* Contain all layer types in an enum

* Implement `as_xlayer` for `LayerType`

* Make id optional

* Rename `ty` to `layer_type`

* Remove `layer_index`

* Move `as_x` functions to tests

* Fix build

This simplifies code and should be better on memory usage since it is
less fragmented.

Also added `width` and `height` attributes to `Layer`.

Closes #54

Parity with tile/object layers

Remove Map::source, add Tileset::source, change Image::source to PathBuf

Layer id defaults to 0 if missing, which is normally unused.
Tile layer name defaults to an empty String if missing, same as object layers.