#41 Allow to manipulate several objects via object control point

[Zoitl Alois]

Currently, there are several object manipulators allowing to change a bunch of selected objects (i.e., move, copy, mulit-copy, mirror, rotate, scale). However the editing process is not intuitive. Several of this manipulations would be faster if they could be done directly in one of the viewports. The manipulations in questions would be. copy, move, rotate, scale, and maybe mirror.

In order to support this it has to be possible that several objects can be selected in a viewport. For these selected objects new object control points have to be shown representing all selected objects (e.g., the center and a surrounding box). This is rather common for 2d drawing tools like Dia.