Be able to add tags/labels to Terrains for sorting

[gvivster]

I nearly always use multiple tilesets with Terrains in them at once, and would like to be able to keep different types of Terrain together visually - for example, walls, floors, water, interior, et cetera.

To that end, it would add huge utility for me to be able to assign labels/filters/groups/etc to individual terrains, and be able to sort terrain sets by those groups if I wish, no matter what specific tileset they come from.

(If I could select multiple terrains and do this in bulk, that would be even better - it would probably cut down the time spent doing it by 10x.)

Building off of this issue: Add ability to reorder terrains #1603 Also somewhat related to this one: For Terrains, a more visually-friendly, more detailed, zoomable grid view #3851