[ENHANCEMENT] Add Script Events for losing/gaining focus

[AbnormalPoof]

This PR adds callbacks to Modules for when the game loses/gains focus. Mods can use these callbacks like so:

import funkin.modding.module.Module;

/**
 * A test of the focus events.
 *
 * Delete this later.
 */
class FocusTest extends Module {
  public function new() {
    super('FocusTest');
  }

  function onFocusLost(event) {
    trace("LOST FOCUS!");
  }

  function onFocusGained(event) {
    trace("FOCUS GAINED!");
  }
}

Issues

• The game fires the FOCUS_LOST event when the game is closed/exited, though I'm pretty sure this is a Flixel bug since the events ride off of Flixel's existing onFocus() and onFocusLost() functions.

[JVNpixels]

This is cool!