Allow selection and movement/rotation of multiple objects

[HaSa1002]

Summary

After some time, it may become necessary to adjust multiple objects in relation to other groups of multiple objects. Right now that's only possible by moving every single object individually.

Proposal

Allow selection or even a selection by drawing a rect of multiple objects, use their bounding box to calculate the centre, move the gizmo there and update all the objects at once.

Alternatives

Listening to motivational music while moving one object at a time while it's your time to shine... (yeah it is inspired by the musical named "Bring it on")

Additional information

Likely a candidate to get implemented alongside #166