Aseprite core tweaks to handle Attachment System tilemaps

[dacap]

The Attachment System requires several tweaks to make some commands work correctly or as expected.

• Attachment layers are (should be) tilemap layers of 1x1 with one tile
  Sometimes it might be possible that some layer has empty tiles at the edges (legacy file?)
• Ctrl+mouse (move tool) should show the tile bounds (not the tilemap bounds), same for Ctrl+T to select the bounds of the tile (Ctrl+T doesn't select the bounds of the attachment (selects the whole tile, even empty pixels) #95)
• Shift+H / Shift+V to flip the whole cel should work as with regular layers (Flipping an attachment #121)
  This might require a customized behavior for the Attachment System because in a regular tilemap we might want to flip the tiles (and then flip the rot/flip flag for tiles / which is not yet supported)
• Start drawing in an empty cel should create a new attachment / assign it and then start drawing in that attachment (Add & Select New Empty Attachment #119)

All this is related in some way and requires adjustments in the core or a possibility to handle special events in the program to make it work as if the attachment layers were regular layers.