Facilitate convenient switching from the TileMap to the new TileSet editor

[snailrhymer]

Describe the project you are working on

None - tidying up a discussion on TileMap improvements found here:

#1769 (comment)

Describe the problem or limitation you are having in your project

Editing TileMaps and TileSets could be more convenient.

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

See below

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

When a user double clicks on a tile source in the TileMap editor, switch to the TileSet editor, with the corresponding tile selected and ready to edit.

Double click in TileMap editor:

Opens TileSet editor with tile selected:

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

Easily worked around by opening the editor yourself. Will likely be used reasonably often.

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

It improves the core TileMap workflow.