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

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.

Start from the editor's File > New actions and the workflow for creating a metatileset from a TMX file. Trace how the currently open TMX is represented and how the existing file chooser selects a source. Done means a new option can create the metatileset from the current TMX without browsing, while the existing workflow still works.

Written by the indexing model from the issue text.