Keymap mod keys: Add warning icon.

That shows when the left and right side key roles don't match.
e.g. The left control key has a different key role than the right
control key.

The warning doesn't actually stop you from setting the modifier
keys though.

Add tooltips on the warning and stop icons explaining the
problem.

Also a couple minor style fixes.
Keymap: static functions back to private methods
Keymap: Center Modifier Keys window in main window
Keymap: Sentence case these pragmas
Keymap: 80 char limit style fix
Keymap: Hide/show warning/stop icon views
Keymap modifiers: make it a floating window