screenshots now use timestamps as name

Instead of being called top_XXXX.bmp where XXXX means nothing,
screenshots are now called YYYY-MM-DD_HH-MM-SS.mmm_top.bmp (and same
idea for bot and top_right).

First obvious consequence, this is easier to manage in a file browser.
Now you have screenshots grouped by time, and you also have the 2 or 3
related screenshots (top, bot, top_right) close one to another.

Another consequence is that there is no need to go through existing
screenshots to find an unused number. Now osGetTime takes care of
everything. And going through files was a very time consuming process.
Now screenshots only need 4 (or 6 if 3D) seconds to be saved (instead of
40 seconds if you had 200 screenshots in your directory already).

Conflicts may happen when people change the date and time on their 3DS,
but that's why I even included milliseconds in the timestamp. People
don't set date and time everyday and they'd be unlucky to take a
screenshot at the exact time, milliseconds included, as another
screenshot right after setting the time one hour earlier...

Source for converting seconds since 1970 (or 1900...) to date and time:
https://stackoverflow.com/questions/21593692/

sysmodules/rosalina/source/menus.c

@@ -25,6 +25,7 @@
 */
 
 #include <3ds.h>
+#include <3ds/os.h>
 #include "menus.h"
 #include "menu.h"
 #include "draw.h"
@@ -176,28 +177,50 @@ void RosalinaMenu_TakeScreenshot(void)
         FSUSER_CloseArchive(archive);
     }
 
-    for(filenum = 0; filenum <= 9999999; filenum++) // find an unused file name
+    u32 seconds, minutes, hours, days, year, month;
+    u64 milliseconds = osGetTime();
+    seconds = milliseconds/1000;
+    milliseconds %= 1000;
+    minutes = seconds / 60;
+    seconds %= 60;
+    hours = minutes / 60;
+    minutes %= 60;
+    days = hours / 24;
+    hours %= 24;
+
+    year = 1900; // osGetTime starts in 1900
+
+    while(1)
     {
-        Result res1, res2, res3;
+        bool leapYear = (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
-        IFile fileR;
+        uint16_t daysInYear = leapYear ? 366 : 365;
+        if (days >= daysInYear)
+        {
+            days -= daysInYear;
+            ++year;
+        }
+        else
+        {
+            static const uint8_t daysInMonth[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
+            for(month = 0; month < 12; ++month)
+            {
+                uint8_t dim = daysInMonth[month];
 
-        sprintf(filename, "/luma/screenshots/top_%04u.bmp", filenum);
+                if (month == 1 && leapYear)
-        res1 = IFile_Open(&fileR, archiveId, fsMakePath(PATH_EMPTY, ""), fsMakePath(PATH_ASCII, filename), FS_OPEN_READ);
+                    ++dim;
-        IFile_Close(&fileR);
 
-        sprintf(filename, "/luma/screenshots/bot_%04u.bmp", filenum);
+                if (days >= dim)
-        res2 = IFile_Open(&fileR, archiveId, fsMakePath(PATH_EMPTY, ""), fsMakePath(PATH_ASCII, filename), FS_OPEN_READ);
+                    days -= dim;
-        IFile_Close(&fileR);
+                else
-
-        sprintf(filename, "/luma/screenshots/top_right_%04u.bmp", filenum);
-        res3 = IFile_Open(&fileR, archiveId, fsMakePath(PATH_EMPTY, ""), fsMakePath(PATH_ASCII, filename), FS_OPEN_READ);
-        IFile_Close(&fileR);
-
-        if(R_FAILED(res1) && R_FAILED(res2) && R_FAILED(res3))
                     break;
             }
+            break;
+        }
+    }
+    days++;
+    month++;
 
-    sprintf(filename, "/luma/screenshots/top_%04u.bmp", filenum);
+    sprintf(filename, "/luma/screenshots/%04u-%02u-%02u_%02u-%02u-%02u.%03u_top.bmp", year, month, days, hours, minutes, seconds, milliseconds);
     TRY(IFile_Open(&file, archiveId, fsMakePath(PATH_EMPTY, ""), fsMakePath(PATH_ASCII, filename), FS_OPEN_CREATE | FS_OPEN_WRITE));
     Draw_CreateBitmapHeader(framebufferCache, 400, 240);
 
@@ -212,7 +235,7 @@ void RosalinaMenu_TakeScreenshot(void)
     TRY(IFile_Write(&file, &total, framebufferCache, 3 * 400 * 120, 0));
     TRY(IFile_Close(&file));
 
-    sprintf(filename, "/luma/screenshots/bot_%04u.bmp", filenum);
+    sprintf(filename, "/luma/screenshots/%04u-%02u-%02u_%02u-%02u-%02u.%03u_bot.bmp", year, month, days, hours, minutes, seconds, milliseconds);
     TRY(IFile_Open(&file, archiveId, fsMakePath(PATH_EMPTY, ""), fsMakePath(PATH_ASCII, filename), FS_OPEN_CREATE | FS_OPEN_WRITE));
     Draw_CreateBitmapHeader(framebufferCache, 320, 240);
 
@@ -229,7 +252,7 @@ void RosalinaMenu_TakeScreenshot(void)
 
     if((GPU_FB_TOP_FMT & 0x20) && (Draw_GetCurrentFramebufferAddress(true, true) != Draw_GetCurrentFramebufferAddress(true, false)))
     {
-        sprintf(filename, "/luma/screenshots/top_right_%04u.bmp", filenum);
+        sprintf(filename, "/luma/screenshots/%04u-%02u-%02u_%02u-%02u-%02u.%03u_top_right.bmp", year, month, days, hours, minutes, seconds, milliseconds);
         TRY(IFile_Open(&file, archiveId, fsMakePath(PATH_EMPTY, ""), fsMakePath(PATH_ASCII, filename), FS_OPEN_CREATE | FS_OPEN_WRITE));
         Draw_CreateBitmapHeader(framebufferCache, 400, 240);