Store grabbed state in Stack

I think this is a bit cleaner than passing the grabbed value through
to the stack especially with wanting to segment the stacks into areas.
Hold click to grab
Replace dice images
Documentation for stack
Remove debug prints
Use mouse pressed not released

This makes it so you can click and "drag" a piece around rather than
needing to specifically click, release, move, click.
Remove redundant escape key check

We already loop over rl.WindowShouldClose() which returns if the close
button is clicked _or_ the escape key is pressed.
initial commit