Allow sampling Terrain in Edit Terrains mode

In the Map Editor, it's possible to pick a Terrain to use by right-clicking a tile corner/edge, rather than from the Terrains list. This is very convenient, especially when using a terrain set with a lot of terrains.

I'd like to see a similar capability for the Tileset Editor when editing Terrains, so that one can quickly pick a terrain by clicking an existing label, without having to go through the Terrains list.
Right-click is already "taken" since it opens the right-click menu. Perhaps right-click with some modifier would work? It wouldn't be consistent with the map editor, but it would better than nothing.

No files, tests, or entry points are identified in the issue. Start by comparing how terrain sampling works in the Map Editor with terrain editing in the Tileset Editor, then determine an interaction that does not conflict with the existing right-click menu. Done means an existing terrain label can be sampled while editing terrains without using the Terrains list.