MythUIHelper: add any base themes to the search path and cache the result

This allows a simple form of theme inheritance where you could add your own
window or widget definitions to a new theme but still use everything else from
the base theme.

First the theme would be searched then any base theme if defined in
themeinfo.xml then the usual fallback to default-wide then default.

The next step would be allow windows and widgets in the new theme to inherit
from ones in the base theme so you would only need to change the elements
you need rather than having to completely redefine them in the new theme.