RFC/Proposal: Fallback code for display screenshot code (used on '/main' Web Remote)

Fixes somewhat #1236 and #1322. Closes #1405 (closed). Possibly fixes #1284 (closed) (the macOS crash), and other reports on Forum of people using negative/offscreen custom postitioning.

This MR adds code that allows a fallback to screenshots in all cases that OpenLP fails to create a screenshot on /main. It does the following:

• Removes the deprecated QApplication.desktop() call (it's removed from Qt6; just passing '0' as winId does the same behavior);
• Adds some utility functions to detect whether the OpenLP is on Wayland compositor, or running on XWayland and Wayland/XCB platform checks;
• Adds grab_screenshot_safe() in DisplayWindow to allow to retrieve a screenshot from window in a Qt-thread-safe manner;
• Adds a _capture_maindisplay_window() and capture_maindisplay_desktop() calls that does the Display Window capture and Desktop capture;
• Adds a workaround to macOS to allow the desktop capture to happen in Qt thread (_capture_maindisplay_desktop_mainthread_safe())
• Automatically fallbacks to Window capture when one of the following conditions is met:
  • Running on Wayland compositor (even when application is running on XWayland);
  • Current service item provides own display (usually presentations);
  • Media is being placed
  • Display custom position is out of bounds of the selected screen
• Adds a capability to allow plugins that ProvidesOwnDisplay to provide a screenshot.
• Reorganizes screen/desktop screenshot calls to grab from the correct screen and with correct sizes (relevant on HiDPI schemes)
• Adds a user setting to allow to always fallback to display window screenshotting mode (disabled by default)

But this approach has some drawbacks when using the Window screenshotting:

• Background videos will not be shown (but there's no easy way on this);
• For the present moment all the presentation plugin that controls external windows don't provide screenshots (so the image will be black on these);
• For the case @ninjakiwi said on #1236 (that it started working on Wayland, I couldn't reproduce it yet although), users would be falled back to Window screenshot code when they shouldn't (unless we find a way to know if it's working, there's nothing we can do on this).

In future, the display glue approach @ninjakiwi said would be a lightier way to do this logic (although I think this should exist alongside screenshoting code for a while).