Add ability to change all instances of a gid by other

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Sometimes I have to reorder the tileset (to reduce some visible artifacts) or add new tiles to one of the tilesets used by the map, and normally I have to redo the map layers that used the old tileset.

I think that the process will be easiest if tiled would add the posibility to change all instances of one gid with another gid.

The first problem is difficult to solve in tiled because as the gids are changed you have to redo the map layers that use the tileset.

To solve the second problem there is a workaround, you can move the tileset to the end of the tileset list (inside tiled), then you can change the image file with the bigger one and the map continues viewing well. But you have to add the new tiles at the end of the tileset.

Start by tracing how Tiled represents tileset global IDs (GIDs) across map layers and how tileset changes are applied. Done means users can replace every instance of one GID with another without manually rebuilding affected map layers, including maps with reordered or expanded tilesets.

Written by the indexing model from the issue text.