add a persistent settings store

src/main/java/wtf/beatrice/autosqueal/config/ScreenArea.java

@@ -0,0 +1,13 @@
+package wtf.beatrice.autosqueal.config;
+
+/**
+ * Which screens the automation is allowed to move the cursor to.
+ */
+public enum ScreenArea {
+
+    /** The main screen only, as the app has always used. */
+    PRIMARY,
+
+    /** Every connected screen, treated as one big virtual screen. */
+    ALL
+}