track groups, find layers inside groups

[jamesalbert]

First off, I love pytmx. Been using it forever and it's awesome. much ❤️

lmk if this is something that's needed. In a lot of cases, I'd find this useful and I noticed an issue (cited below) echoing the same. This PR is an initial attempt to track groups and parse the layers within them.

Changes:

• tile layers (and every other layer type) are found, not just those at the root
• you can find a group with get_layer_by_name as an issue pointed out did not return groups Layer groups do not appear to be working #124
• new layer type: TiledGroupLayer

Tested with:

• empty, populated, and nested group layers in tiled locally
• passed unittests

The group layer object I implemented is pretty rudimentary, idk if we want it to contain a list of layers inside or transitively pass visible from groups to layers. I figured with feedback I could add that if we want to continue down this route. Or this is complete garbage and we can just close 😄

[bitcraft]

This looks good! Thanks for your kind words and contribution.