Major UI changes - including new icons!

- New UI icons for the tools, layer and frame buttons.
- Removed clone, remove and move frame buttons. You can now right click on a frame button to do these actions instead.
- Added first, previous, next and last frame buttons to the timeline.
- Added rulers for the main canvas viewport.
- Window size bumped to 1152x648.
- Default FPS is now 6 instead of 1.
- Fill tool renamed to Bucket.
- Mouse default cursor shape for the canvas is the arrow instead of cross.
- Mouse default cursor shape for the mirror and onion skinning buttons is the pointing hand.

Scripts/HorizontalRuler.gd

@@ -9,6 +9,7 @@ var minor_subdivision := 5
 var first : Vector2
 var last : Vector2
 
+# warning-ignore:unused_argument
 func _process(delta) -> void:
 	update()