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.

Fixes #72

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

Closes #99. ldtk seems to output tiled files with no newlines in the csv
data. Tiled itself does always seem to put the newlines in however, it
can open the ldtk files so we should too.

And not, for example, in pixels

So that it can be more easily displayed to the user if needed

Layer and ObjectGroup are parsing "visible" string, but not object.
TMX docs indicate expected to be "0" or "1"
https://doc.mapeditor.org/en/stable/reference/tmx-map-format/#object