fix(exploit): fix group force attack exploit when a building group is added to a unit selection

[roossienb]

• Resolves Units and Buildings can be grouped together by using the SHIFT key feature #101

While EA fixed the group force attack exploit ("scud bug") when one was trying to add a building to a group with units, it was still possible to do the reverse: Add the building to a group. Select a bunch of units and than add-select ('shift') the group to the current selection.

To remedy this, every object in the group needs to be checked for kindof building when getting added to an existing selection.

[roossienb]

Removed the dependency and have taken the logic out of the for-loop.
Behaviour is now also in line with gentool