[mythtv] Auto-detect UI painter

[Lawrence Rust]

I note that commit fb45eb9 automatically selects the UI painter,
removing the ThemePainter setting.  On Windows this will default in
order to D3D9, OpenGL and finally Qt.

In itself this is a good thing but I have noticed on several Windows
systems that although D3D9 is present and can be initialised that it
sometimes doesn't draw the UI correctly, often resulting in an empty
window.  This is most noticeable on older h/w and systems pre Windows
Vista.

In consequence, I believe that removing the painter setting from the
normal users control could end up with some unusable Windows systems.
The only workaround is to add "-O UIPainter=Qt" to every invocation.

Personally, I can't see what's wrong with keeping the current UI
control, defaulting to auto, and allowing the user to force Qt
permanently if they need to.

-- Lawrence