libmythui: Revisit window/appearance settings

[mark-kendall]

The current settings are confusing and do not necessarily work together. This is largely due to trying to handle fullscreen (see #232) and a windowed mode.

Proposal:-

1. with libmythui: Fullscreen should always mean fullscreen #232 in place, clearly differentiate between settings that are for windowed mode and fullscreen. (e.g. window border)
2. when running in a window, disallow some behaviours e.g. all display mode switching (which has numerous issues when running in a window and is probably not appropriate to a desktop window).

see also #233