* 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

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.

Move parse functions to its respective types, rename `Colour` for consistency with PropertyValue::ColorValue, add documentation to map members, add Map::source, misc improvements