Fix cursor visibility API compatibility with newer GNOME Shell versions

[neoRandom]

Fixes #53

Summary

Adds fallback cursor visibility handling for GNOME Shell versions where Meta.CursorTracker.inhibit_cursor_visibility() and uninhibit_cursor_visibility() methods are not available in the JavaScript bindings.

Changes

• Added runtime check for cursor visibility methods availability
• Implements fallback using global.stage.set_cursor_type() when tracker methods unavailable
• Maintains full backward compatibility with versions that have the methods
• Preserves input focus inhibition via ClutterSeat unchanged

Technical Details

The fix uses a dual-path approach:

1. Primary path: Uses Meta.CursorTracker methods when available (original behavior)
2. Fallback path: Uses global.stage.set_cursor_type() when methods not found
   • Hide: Sets cursor to Clutter.CursorType.NONE
   • Show: Sets cursor to Clutter.CursorType.DEFAULT

The implementation gracefully degrades without crashes on any GNOME Shell version.

Testing

Please verify the extension:

• Works on GNOME Shell versions with the tracker methods
• Doesn't crash on GNOME Shell versions without the methods
• Cursor visibility controls function as expected