Support is largely copy-pasted from, and tests are shared with, the qt implementation (qt_compat._maybe_allow_interrupt, #13306), the main difference being that what we need from QSocketNotifier, as well as the equivalent for QApplication.quit(), are reimplemented in ObjC.

qt_compat._maybe_allow_interrupt is also slightly cleaned up by moving out the "do-nothing" case (old_sigint_handler in (None, SIG_IGN, SIG_DFL)) and dedenting the rest, instead of keeping track of whether signals were actually manipulated via a skip variable.

Factoring out the common parts of _maybe_allow_interrupt is left as a follow-up.

(Test e.g. with
MPLBACKEND=macosx python -c "from pylab import *; plot(); show()" followed by Ctrl-C.)

Closes #3991 (which was closed as cantfix in #3991 (comment); likely much of #4006 can now be reverted).

Also closes #10002 by providing _macosx.stop as a private function to call [NSApp stop]; whether we want to make that public is another question.

PR summary

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines