Add Eraser Tool

[pexeixv]

Goal:
Allow users to erase pixels back to white.

Requirements:

• Toolbar toggle between "Paint" and "Eraser" mode.
• Clicking cells in Eraser mode resets them to #ffffff.
• Must integrate with Undo/Redo.

Notes:
Improves user control over mistakes and corrections.

[AskitEndo]

@pexeixv I could give this a try.

[AskitEndo]

@pexeixv I'll be adding eraser and pencil icons both and both with shortcuts 'E' and 'P' respectively, although eraser has deafult color white, more color can be selected, 90% through the changes, will PR soon.