Don't allow themed buildings from imported themes grow in non-themed districts by default

[bloodypenguin]

[boformer]

basically that would mean:

Select "city-wide" district
Select map-native theme
Enable "Allow Buildings which are not in any theme"

The code has to be added here: https://github.com/boformer/BuildingThemes/blob/master/BuildingThemes/BuildingThemesManager.cs#L579-L602

The "Enable Theme Management in this district" should still be disabled (so that the configuration is not saved in the xml save).