This PR adds support for displaying the content of the GPv3 object in the outliner.
* Displays the grease pencil data, layers and layer groups.
* Data and layer groups have the total layer count displayed next to them. Layer groups also have hierarchy lines.
* Active layer is highlighted and can be changed by clicking on a layer.
* The visibility of layers and layer groups can be toggled by clicking the eye icon.
Note: The icon for layer groups is temporary.
Pull Request: https://projects.blender.org/blender/blender/pulls/111105
310ccbdc33