Tilemap features for first stable v1.3

[dacap]

For a first v1.3, we should need the following items from #977:

• Export tileset (UI and CLI), we've used https://github.com/dacap/export-aseprite-file so far
  • Export tilemap
• When a sprite contains tilemap layers, the sprite properties dialog should show a section of "tilesets":
  • We should be able to reorder tilesets
  • We should be able to add new tilesets
• Copy and paste tiles in the color/tiles bar doesn't work
• Painting in manual mode in non-zero empty tiles (tiles with a non-zero index but with all pixels=0) doesn't work (?)
  • It looks like it does work, so I'm not sure if there is a special case were it stops working
• Painting in manual mode clears empty tiles with non-zero index and undo doesn't work Tilemap Duplication and overly aggressive merging  #3351

All other features will appear progressively in the v1.3.x series.