Account for full screen and dynamically changing modes inside QScreen::availableGeometry()

When an application goes into full screen mode, then it is possible that QScreen::availableGeometry() will still return a size that is excluding any menu area or dock area. This should adapt to the fact that it is in full screen mode and any other possible modes that can happen on the desktop.

Using macOS as an example:

• The presentation options for the application can change how much space the screen has available, so it should take this into consideration
• If the application is in full screen mode then the dock and menu are hidden so that should be accounted for too