fix scheduling, threading and movement bugs

scheduling:
- cursor mover created a new java.util.timer for every movement and never
  cancelled it, leaking a live thread roughly every 10 seconds; timers are
  replaced by a single shared scheduledexecutorservice with daemon threads,
  shut down when the automation stops.

clicks:
- the double click sequence used thread.sleep() inside a timer task, blocking
  the scheduler thread; it is now scheduled as one-shot actions on the shared
  scheduler, with the same pacing as before.
- stopping the automation now also cancels in-flight movements and pending
  clicks, instead of letting them run to completion.

movement math:
- when the destination shared one coordinate with the starting position, the
  step on the other axis was computed as length/0 = infinity, and the stop
  condition checked one axis with || instead of &&, so a movement could stop
  (and click) before reaching its destination. steps are now guarded and the
  movement stops only when both axes have converged.

ui and lifecycle:
- the app could not be closed: closing the window only hid it, and ESC
  unregistered the hotkey without exiting. the window now exits cleanly on
  close, stopping the automation and unregistering the native hook.
- toggleRunning() was called from the jnativehook thread and touched swing
  components off the EDT; UI updates are now marshalled to the EDT.
- the hotkey thrashed: key repeats re-fired the toggle while ctrl+alt was
  held, and any other key pressed with the combo held toggled it too. it now
  fires exactly once, when the combo is completed.

away-detection (CursorMoveListener) stays intentionally unscheduled: it cannot
tell the user's movements apart from the app's own, and will be reworked in a
future iteration.

src/main/java/wtf/beatrice/autosqueal/Main.java

@@ -7,6 +7,8 @@ import org.apache.logging.log4j.Logger;
 import wtf.beatrice.autosqueal.listener.KeyPressListener;
 import wtf.beatrice.autosqueal.ui.MainWindow;
 
+import javax.swing.*;
+
 public class Main {
 
     private static final Logger LOGGER = LogManager.getLogger(Main.class);
@@ -16,7 +18,9 @@ public class Main {
         LOGGER.info("Hello world!");
 
         registerJNativeHook();
-        mainWindow.init();
+
+        // all Swing components must be created and updated on the EDT
+        SwingUtilities.invokeLater(mainWindow::init);
     }
 
     private static void registerJNativeHook() {
@@ -34,8 +38,19 @@ public class Main {
         }
     }
 
+    public static void unregisterJNativeHook() {
+        try {
+            GlobalScreen.unregisterNativeHook();
+            LOGGER.info("Successfully unregistered jnativehook library!");
+        }
+        catch (NativeHookException ex) {
+            LOGGER.error("There was a problem unregistering the native hook.");
+            LOGGER.error(ex.getMessage());
+        }
+    }
+
     public static MainWindow getMainWindow() {
         return mainWindow;
     }
 
-}
+}