Make metatileset creation more user-friendly by adding "New tileset from current TMX" to "File > New" #3387

I make a lot of metatilesets. When I do, it's a pain (especially since the TMX is almost always already open!) to have to hit "browse" and hunt down the TMX in the file explorer, especially since it shows "image files" by default and so we have to manually choose "tiled map files" instead, every single time. (I often go into the "save" window, copy the file path and paste it into the "browse" file path, just to trim a dozen clicks off of the workflow... doing that is not much less of a pain.)

Instead, it would be great to have an option to simply make a tileset from the currently-open TMX. That would make it a few simple clicks, instead of lots and lots of clicks to "find" a TMX file that you're already looking at.