Draw Grid only over Canvas. Added PixelGrid. (#427)

* Draw grid only over the Canvas (when in tiling mode)

* Replace some magic numbers with enums.

It's too easy to break something when adding something new in here. Should be a little harder now.

* Added Pixel Grid.

- Pixel grid is shown only when it's enabled and camera is zoomed close enough.
- Settings: pixel_grid_show_at_zoom (as a percentage because that's what's shown in the settings panel), pixel_grid_color. Default values might need changing.
- To distinguish between grid and pixel grid default settings for grid width, grid height are changed.
- Now both grid and pixel grid are drawn above (after) tile mode. Grid is drawn above (after) pixel grid.

src/UI/Canvas/Canvas.gd

@@ -10,6 +10,7 @@ var sprite_changed_this_frame := false # for optimization purposes
 
 onready var currently_visible_frame : Viewport = $CurrentlyVisibleFrame
 onready var current_frame_drawer = $CurrentlyVisibleFrame/CurrentFrameDrawer
+onready var pixel_grid = $PixelGrid
 onready var grid = $Grid
 onready var tile_mode = $TileMode
 onready var indicators = $Indicators