[Pieces] Initial editing tools

[mbrown1413]

Currently, the only way to edit a piece's voxels is left-click to add and right-click to remove. This can be cumbersome at times.

Adding different tools that provide editing modes will make editing much faster in different circumstances. Some examples of tools we may want to implement:

• Touch - Clicking will add/remove, dragging moves the camera (this is what is currently implemented)
• Box-drag - Drag a box around the voxels to add (left-click) or remove (right-click)
• Pencil-drag - Dragging will add/remove any voxels in the drag path
• Toggle drawing in
• Erase (toggle) - Reverse left/right click for addding/removing voxels.
• Span Axis - Drawing a single voxel will act like you've drawn on all voxels in the direction of one axis. Like the BurrTools "Toggle drawing in all x layers" button.
• Mirror - Mirror all edits across an axis

The tool concept will also be useful for future features, such as:

• Variable voxels - Mode to paint variable voxels instead of solid ones
• Voxel attributes - "Select" mode to select an individual voxel and edit its attributes. Same for a voxel's sides

The exact scope of this issue is to:

1. [ ] Determine an initial set of tools to implement
2. [ ] Create issues for tools we want to implement in the future
3. [ ] Implement the initial set of tools

[mbrown1413]

I've implemented box select (da66325c083c85f7729d9213be76425b4edd89c3) in the 2D view. There aren't any buttons to toggle it since you can still individually click voxels. We can already toggle optional voxel drawing.

All other tools will be done later:

• Eraser tool: #80
• Span axis and Mirror: #79
• Pencil drag: No issue for this. I'm not convinced this is useful, so we'll wait until somebody comes up with a good enough use case.