Compositor: Implement shortcuts for Viewer nodes

Artists often want to quickly switch back and forth between two or more nodes while compositing.

This patch implements two operators `NODE_OT_viewer_shortcut_set` and `NODE_OT_viewer_shortcut_get` that allow users to map a viewer node to a shortcut. For example, pressing `cltr+1` while a node is selected, assigns that node to the shortcut `1`, creates a viewer node if it has none attached and sets that viewer node to active. Pressing `1` will set the active node with shortcut `1` to active.

Shortcuts are saved in DNA to preserve them after saving/loading blend files.

Limitations:
- Only compositor node tree is supported, because shading editor has no viewer node and geometry nodes viewer works differently.

Pull Request: https://projects.blender.org/blender/blender/pulls/123641