Correctly handle commands for multiple units

[Itms]

When there are multiple units in the selection, we're facing several problems, which arise from the fact that the GUI uses the first unit of the selection (this first unit being the one with the lowest entity ID).

Problems include:

• The back-to-work button does not always show up (workaround included in r17880 and reverted afterwards)
• #2205
• #3302
• Building structures when female citizens and soldiers are selected together

and possibly others.

The unit commands would probably need a big overhaul if we want to fix that issue without yucky workarounds like r17880.