Revert "Revert "scide: on Mac, make one global menu to share by all windows""

This reverts commit 2ea728a51c37a819cd2af58a2a0d2b2e9dc0c295.
scide: prevent erroneous overriding of shortcuts on Mac OS

On Mac OS, for some global menu items, there is a ShortcutOverride event
with key() == 0, which seems like a Qt bug. Text widgets accept all
ShortcutOverride events with key() < Qt::Key_Escape, which includes 0.
This patch ignores such erroneous ShortcutOverride events on global
application level.