New TrainAR Object Conversion Window + new mesh decimation algorithm choice

[jblattgerste]

This Pull Request implements the following changes:

• Always forces the Gizmo Handle to “Pivot” when switching to the TrainAR Authoring Overlay (as to the feedback from evaluations that displaying the center in the authoring tool, while TrainAR uses the pivot, is misleading)
• Implements an entirely new conversion preview window (better interactive preview, display options, options to move the pivot point of the center of an object)
• Reworks the way objects are processed. Now objects to be simplified are: Instantiated as a copy, their meshes are combined on opening the tool, they are then simplified/named/pivot-moved and afterward the copy is copied into the scene and the original object deleted.
• Implements a new simplification algorithm as the first choice: vcglib Tridecimator

If somebody would review this:

• Does this have unwanted consequences in the authoring tool?
• Did create memory leaks with all the mesh instantiating for the previews?
• Is this still (beside some additional explanations + a new picture, which I will add after merging) consistent with the documentation?
• Does this still properly deploy to Android/iOS or has some performance/build implications? (I had to enable another .Net backend to get dynamic code working, but I am unsure if this might cause problems on build/deployment)

[jblattgerste]

So one bug i am aware of: If you start the object conversion window for the first time, there is a null pointer because it searches for an instance of the conversion window.