Fix possible segfault when exiting a screen when using the Qt painter

This changes MythScreenStack::PopScreen() to call deleteLater() on the
screen rather than immediately deleting it so as to allow any keypressEvent
handlers to complete safely before the screen is deleted when control
returns to the event loop.

Fixes #11383.
(cherry picked from commit c713988707d2c795a2a2abeff817d1b187d4b3f6)