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.

project.godot

@@ -429,6 +429,11 @@ right_pan_tool={
 "events": [ Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"alt":true,"shift":false,"control":false,"meta":false,"command":false,"pressed":false,"scancode":77,"unicode":0,"echo":false,"script":null)
  ]
 }
+show_pixel_grid={
+"deadzone": 0.5,
+"events": [ Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"alt":false,"shift":false,"control":true,"meta":false,"command":true,"pressed":false,"scancode":72,"unicode":0,"echo":false,"script":null)
+ ]
+}
 
 [locale]