Random object adding

[gajop]

When adding objects, randomly pick them from a group instead of using specific ones.
For example, the user may want to add "trees", without specifying concrete unitdefs.
The UI should randomly pick the concrete unitdef, based on defined random distributions (may be best to make it uniform for starters).

Also see feature-placer's feature groups: https://github.com/Spring-Helper-Projects/FeaturePlacer/blob/master/LuaRules/Configs/randomfeatures.lua

[gajop]

We may also allow multiple different groups to be added at the same time.
For example groups of 1) trees, 2) shrubs/grass, 3) rocks could be added at the same time.