Feature request: hide maps in Porymap's UI

[ketsuban]

Deleting a map is nontrivial—the standard advice (such as in the "How to delete a map" tutorial on the pokeemerald repo) is to merely remove references to a map, rather than delete it and have to find all the places it was referenced in the code—but this leaves Porymap cluttered with vanilla maps you probably don't want to interact with and makes it hard to see and work with your own maps. It would be nice to be able to hide maps in Porymap's UI to remove this clutter.