GPv3: Python API additions to layer, layers, layer_groups
This PR contains several additions to the GPv3 Python API for
`layer_groups`, `layers` and `layer[key]`.
**Layer groups**
`grease_pencil.layer_groups`
- `.new(name, parent_group=None)`
- `.remove(layer_group, keep_children=False)`
- `.move(layer_group, 'UP'/'DOWN')`
- `.move_top(layer_group)`
- `.move_bottom(layer_group)`
- `.move_to_layer_group(layer_group, parent_group)`
**Layers**
`grease_pencil.layers`
- `.new(name, set_active=True, layer_group=None)`
- `.remove(layer)`
- `.move(layer, 'UP'/'DOWN')`
- `.move_top(layer)`
- `.move_bottom(layer)`
- `.move_to_layer_group(layer, layer_group)`
**Layer**
`grease_pencil.layer[key]`
- `.select`
- `.lock`
- `.matrix_local`
- `.matrix_parent_inverse`
- `.parent_group`
Pull Request: https://projects.blender.org/blender/blender/pulls/121797
180474aaee