Improve button UI for interaction states (active/down, focus, disabled)

Due to the "stately" nature of these changes, a screen-shot of these changes won't suffice.

This pull adds
* the standard "focus" state indicator of a dotted outline (in the button's color).
* an "active" (or down) state that overrides the hover state -- perhaps this can be further improved.
* a cursor indicator for disabled buttons