Extend texture projection / decal layer feature

*ArmorPaint 0.8-811b5fc, Windows*

**Feature description:**
### Extend texture projection feature / make it more usable
In the current version, the texture projection mode is very useful. Here is what I suggest for improvement:
- currently there is no possibility to place textures in the 2D viewport (this would be very helpful).
- scaling and rotation can be done manually, but because there is no snapping it is difficult to adjust precisely (snapping would be great - e.g. while scaling/rotating/moving by mouse click, press Ctrl. at the same time)
- input fields, where the values are to be seen / one can manually change the values for rotation, scaling and transform.
- select your own pivot (e.g. on middle mouse button)

The issue names no files, tests, or entry points. Start by locating the texture projection or decal layer implementation in ArmorPaint, then determine how the requested 2D viewport placement, snapping, transform fields, and custom pivot should be scoped; done means the selected projection can support these interactions.

Written by the indexing model from the issue text.