Allow a SimpleComboBox to be set to nil.
Add tests to allow SimpleComboBox to be set to nil.
Remove unnecessary set of active_iter.
Change EditFactoryWidget over to SimpleComboBox.
Convert EditExtractorWidget over to SimpleComboBox.
Decided I don't want to force-abandon the planet when the user sets the planet type to "uncolonized". It's causing too many weird little load
bugs. If you set it to uncolonized, you'll just have to deal with the ugly icon as your punishment, but otherwise add/remove buildings as you
see fit.