When first opening the picker window, we attempt to find a valid class and type to select. If the picker window was closed with filters enabled, there may not be anything list that is usable.
Resolve this by using callbacks to find the first usable type when no types are listed.
This allows ctrl-click on a type in a build-picker window to remember it
as a favourite. An new filter button to show only favourites makes it
simpler to use these types.
Favourite types are saved locally in favs.cfg, so are remembered between
games.
This filters the build-picker type lists to only show types that have
already been placed in the current game, making it simpler to get to
build matching features.
Toggling the "All" filter causes the class selection to be ignored, so
that items from all classes can be displayed together. The class text
filter is still applied.
This makes it easier to search amongst types for a feature.
These windows now share a common code base for choosing and display class and types.
An additional text filter is added to search types by name instead of just classes.
