Add a way to pin the Voxel Graph Editor

[rcorre]

Some builtin editors in Godot, like the AnimationPlayer editor, allow you to "pin" the editor so it is always visible, even if you select a different node. It would be nice if the Voxel Graph panel could be "pinned" in the same way. When working on a graph-based world, I often switch to different nodes (especially the instancer), and it is a bit of a pain to navigate back to the graph and get it open again.

[Zylann]

Wonders why this has to be coded in every single editor using the bottom panel container

[Zylann]

Added in f81e64d

[Zylann]

I got distracted and made the editor undockable too

2022-08-02.21-51-16.mp4

[rcorre]

Neat! I'm not sure if I'd ever want to undock it, but I can confirm the pinning works as expected in the latest version. Thanks!