Implement cocoa clipboard support.

Implement cocoa clipboard support.

Add QCoccoaClipboard which wraps the existing
QMacPasteboard implementation. Remove unused
QClipboard integration code from qmacclipboard.mm

Change mime type cleanup from using qAddPostRoutine
to using an explicit call to destroyMimieTypes in
the cocoa platform integration destructor. This is
necessary to ensure cleanup happens in the correct
order on app shutdown.

Change-Id: Ief0e0d996b04c8e84e9fd2cd3a17fb5bd73bb761
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>