state: fix base mod set/clear behavior

state: fix base mod set/clear behavior

This commit fixes the incorrect current behavior, where at the end of the
following key sequence
Left Shift down, Right Shift down, Left Shift up
the Shift modifier is cleared.

Clearly the code is not as nice as before, but it seems like some count
of the depressed modifiers must be kept.

The code is lifted mostly as is from xkbActions.c. [ There they also
assign to setMods and clearMods each time and not OR it. I assume its
correct, although I wouldn't have guessed... ]

Signed-off-by: Ran Benita <ran234@gmail.com>