Add a new 'stack' mode

[kayaulai]

Right now, it is confusing that you can create stacks in rez and track mode, but not in stack mode.

Solution: Add a new Stack mode. In this mode, you:

• Cannot create tracks and trails
• Cannot create rezzes and resonances
• Blank chunks could go either way - I'd prefer if it's possible, though we could also disallow them if it's too hard.

The new mode should have a logo, perhaps of stacked rectangles.

[johnwdubois]

I agree.