The extension manager starts dialogs from its own thread. But the
OSX backend currently doesn't defer these calls to the main thread.
This implements the deference by running the called function
in the main thread, using a code ^Block, and returning the result
via a SalYieldMutex member.

Change-Id: Id8977991e3eda91da27c23d8021e028d4f4cefe5
Reviewed-on: https://gerrit.libreoffice.org/42448Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>