This implements layer parenting and layer transforms.
* Adds a new "Transform" panel in the object-data properties with the (local) translation, rotation and scale.
* Adds a new "Relations" panel with the parent property (and also bone name in case the parent is an armature).
* When converting from GPv2 to GPv3, the parent and transforms are converted too.
* Bone names are updated if they are renamed in the armature.
Implementation details:
* The positions in the drawings are always in layer space. During extraction, we transform the positions to object space. Note that this could be optimized further and done in the render engine itself.
* This means that e.g. the selection code (which needs to know where the positions are on screen) now takes this transform into account.
* The layer transform is calculated when accessed (from the location, rotation, scale properties).
* Code that needs to know where the positions are on screen now takes this new transform into account.
Pull Request: https://projects.blender.org/blender/blender/pulls/117247
0a45acbe3b