Allow bottom panels to persist after selecting a different node

[eisegesis-623]

Describe the project you are working on

A third-person 3D character controller with complex animations

Describe the problem or limitation you are having in your project

Every time my AnimationTree node is no longer selected, the AnimationTree editor closes and is reset to the main view. This can become very frustrating in more complex projects, where I would like to just have the panel open while working on related tasks.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

While I'm specifically concerned with the AnimationTree tab, a lot of the Bottom Panel tabs are dynamic like this, so I propose an option to "pin" them so that they can persist even when Godot would normally auto-close them. The functionality would be similar to how the Animation tab works, where it persists until replaced by another tab.
These do not need to persist when switching to another scene entirely, of course. But having them persist in each scene individually would be nice.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Add a "Pin" button to the corner of the panel to allow the tab to persist.

If this enhancement will not be used often, can it be worked around with a few lines of script?

N/A

Is there a reason why this should be core and not an add-on in the asset library?

Animation workflows are a core part of game development and the editor be designed to make it as easy as possible.