Add support for scaling objects

[Qwlouse]

It should be possible to scale objects during import. This includes:

• [x] adding a scale parameter to Objects
• [x] add scaling to Simulator
• [x] add scaling to Blender viewer / interface
• [ ] adjusting physical information such as mass, volume, area, inertia, ...

[Qwlouse]

Partially solved in #54. Objects do have a scale parameter that is synced to Blender. However, support in the simulator is partial only, since pybullet only supports uniform scaling at import time. Thus currently you cannot:

1. rescale objects that were added already
2. scale objects non-uniformly (so for example stretching along the X-axis only). Fixing this is rather tedious, since for 1. we would have to delete and re-import the object everytime the scale is changed. And for 2., we would have to manually edit the mesh.