Allow removing Terrains from Tilesets on export

Nobody has claimed this yet.

Terrain labels are very rarely useful outside of Tiled, and are thus often a waste of bytes for production TSX/TSJ files. For example, a 17KB TSX I have becomes 4KB if I remove the `` element.

An export option to remove all the terrain data would be a convenient alternative to removing the terrain sets manually.

Tangential issue: There's currently very little benefit to exporting Tilesets as no maps will be set to use the exported tileset file, requiring a post-export step to update the tileset references. It would be helpful to be able to automatically *replace* tilesets with their exported version in Maps on export somehow, e.g. by optionally assigning replacements for each Tileset that would be used if an export option like "replace tilesets on export" is used.

Start by tracing the tileset export path for TSX and TSJ files and how the element is written. Done should include an export option that omits all terrain data; the separate idea of replacing tileset references in maps should remain separately scoped.

Written by the indexing model from the issue text.