Update Tileset Editor manual page

The Tileset Editor is a relatively key feature of Porymap, but in the manual we only briefly explain parts of the window. It could use a bit more of a walkthrough on how it's used, with a gif or two showing the workflow of selecting and editing a metatile. There are also a handful of new features that aren't mentioned, and it's worth mentioning Porytiles as an alternative in here.