libmythui: Fullscreen should always mean fullscreen

[mark-kendall]

The main UI should always use a fullscreen widget/window if fullscreen is expected.

Currently, if the window size is adjusted (for overscan etc), then the window is resized and re-positioned.

It should still use a fullscreen widget but render into the visible (smaller) area.

This would fix display size adjustments where only fullscreen is supported (e.g. Android, linux eglfs/no x) and avoid a number of windowing issues that result from moving/resizing the window (a particular problem on macOS).

It would also allow the MythScreenWizard to work properly without having to launch another process - and removal of sundry UI code that is in place purely to workaround issues with launching a second UI process.

See also #227