Converting a tilemap to regular layer and converting it back makes the old tileset appear

Probably related to #78

Steps:
1. Create a new sprite with transparent layer (200x200)
2. Select all (Ctrl+A) and *View > Grid > Selection as Grid*
3. Draw something in the Layer 1
4. Convert Layer 1 to tilemap (right-click > Convert to > Tilemap)
5. Then use "Setup Sprite" in the Attachment System
6. Convert the layer to regular layer
7. Convert the layer to tilemap again

You will see something like this:


Example script:
```lua
local spr = Sprite(200, 100)
spr.gridBounds = Rectangle(0, 0, 200, 100)
app.command.ConvertLayer{ to="tilemap" }
app.command.ConvertLayer{ to="layer" }
app.command.ConvertLayer{ to="tilemap" }
app.command.AttachmentSystem_SwitchWindow()
```
After that we "Setup Sprite", convert the layer to a regular layer, then a tilemap layer, and the issue appears (there are other crashes, but that is other issue that will be solved in the Aseprite core)