Support Mac key equivalent Cmd+Period

On Mac Cmd+Period isa special key combination which never got delived
to Qt application. We can intercept these special keyboard shortcuts in
the performKeyEquivalent function.

Task-number: QTBUG-11386
Change-Id: I680385bde07b2810e8bde86ec9fbbe7e09156c84
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>