Add wxEventLoop::ScheduleExit().

Add wxEventLoop::ScheduleExit().

This method allows to request exiting from the given event loop even if it's
not the currently active one, unlike Exit() which would assert in this case.
With it, it becomes possible to ask the loop to terminate as soon as possible
even if a nested loop is currently running.

See #10258.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74335 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775