macosx: use a dispatch queue to notify extensions about modified inputs

This notifies the extension manager on the non-main thread as before,
the queue ensures that changes in quick succession are processed in-order.
For the rationale see a8ff52782d21b6425d8c26c96a2269939ac6ce18.

Fixes another deadlock on application termination.

close #10647
macosx: properly inform the extension manager about finished inputs
macosx: cosmetics
macosx: remove dead code from extension manager