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/SecondViewport.gd

@@ -2,4 +2,4 @@ extends Viewport
 
 # Called when the node enters the scene tree for the first time.
 func _ready() -> void:
-	world_2d = $"../../ViewportContainer/Viewport".world_2d
+	world_2d = Global.canvas.get_parent().world_2d