Improve editor dock dragging usability

[lodetrick]

Describe the project you are working on

Godot :)

Describe the problem or limitation you are having in your project

See #12430

Currently dragging docks between areas is not a very good user experience. It does not tell you visibly whether a dock is allowed in an area other than a small crossed out circle by your mouse, and the actual area to place the dock is small, only in the tabbar.

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

Allow docks to be dropped in the entire TabContainer area, and add a highlight for when docks are allowed (ie if they area allowed to be horizontal in the bottom dock).

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

You can drag between docks and drop anywhere inside the TabContainer (below), and eventually when the bottom dock is added to the dock system there will be an error color telling you that the dock will not be accepted there.

Screen.Recording.2025-05-23.at.2.51.05.PM.mov

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

No, this enhancement is related to a core system.

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

Docks are a core system and any significant increase in usability to them should be core.