handling cmd-. in plugins

apple provides "cmd-." as "cancel operation", which has some specific semantics. especially it calls NSResponder-cancelOperation on the QNSView. the 'standard' way to handle cmd-period in qt applications is to use a native event filter (compare https://github.com/supercollider/supercollider/blob/master/QtCollider/hacks/hacks_mac.mm), but native event filters cannot be used from within a plugin context.

qt should provide a way for users to handle cmd-. in qt-only code.