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

fixes #47

Fixes #41

Not sure if the method is optimal, but here is some tile-flippin' goodness.

Also add test for parsed image layers.
The test can be also used as an example off how one can get data from
the parsed map.

this just adds a field to the Tile so that data may be parsed and accessed