Offload Some Tasks To Secondary Thread

[IAGOnGithub]

Most, if not all, tasks seem to run on the same thread. While this is fine in many cases, some intensive tasks will cause the GUI to lag significantly as rendering is delayed. Ideally, this shouldn't happen. Some tasks should be offloaded to a separate worker thread to let the main thread (where application rendering takes place) run without significant interruptions.

This issue might be a duplicate. I searched "thread" in the issue tracker and nothing came up.

[GriffinRichards]

What was causing Porymap to lag significantly? (I'd be surprised if there wasn't lower-hanging fruit to address before we'd want to turn to threading)

[IAGOnGithub]

Bulk selecting metatiles is the big one, and then (sometimes) when editing tilesets.

[GriffinRichards]

👍 Selecting metatiles from the map is about as inefficient as it can be (it's creating and rendering an entirely new set of images for every update of the selection rectangle) so there's definitely some easier ways to improve the speed there.