Add ability to move/resize floating windows beyond the region boundary.

Add ability to move/resize floating windows beyond the region boundary.
Windows will change regions based on the cursor location or, if being
moved by a key binding,  window center.

Add 'soft boundary' window containment behavior to region boundaries.
When moving a window past the region boundary, the window will 'snap' to
the region boundary until it is moved more than boundary_width distance
beyond the edge.

Add constraint to ensure windows always have at least 1 pixel in a
region.

Add new boundary_width configuration option.
Disable the window containment behavior by setting this option to 0.

Fix floating window stacking order issue on multiple-region setups.

Eliminate unneeded region parameter on load_float_geom() and
store_float_geom().