On some systems, brackets cannot be used as event details
(they don't have a keysym), so use +/- instead (both on
keyboard and keypad) and add ctrl-= as a synonym of ctrl-+
for convenience.

[sp: Had to change accelerator to show only "$M1T-="; the
     original version included "$M1T-+ $M1T-=" but this is
	 not drawn at all on Mac OS X.]
Signed-off-by: NMichele Ballabio <barra_cuda@katamail.com>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>