* Change from storing layers by index to Dictionary key
Models are now stored with the `TiledMapLayer` they are part of.
* Allow render calls via TiledMapLayer
* Add TiledMapGroupLayer and model build recursion for groups
* Add rendering support for TiledMapGroupLayer
* Update TiledMapReader to read TiledMapGroupLayers
* Update Features Demo to Include a Map with a Layer Group
* Update TiledMapWriter and TiledMapProcessor to Support Groups
* Update TiledMap to Report Layers Correctly
* Demo.SpaceGame working
* Delete unused files, update icon
* Demo.StarWarrior building
* Removed unused files
* Demo.Features Collision
* Platform Demo Building
* Nuclex Gui Project
* got the old platformer demo running but it's still missing stuff
* Demo.Features added to solution
* Demo.Features building
Currently, most demos don't build and were excluded from the project.
* Demo.Features.NetCore building
* Demo.Features building and running
* First demo usable
* Close button
* Tiles + Sprite Demo
* More demos working
* Fix Back() for new Gui
* Remove old project files
Added comments into the importers and processors to show off what the new code does. Removed unneeded code involving locating external files as `Content` handles this for us automatically.
Added the remaining features of the .TMX Tiled Map format. Removed the parts of the format which aren't needed for rendering, like `TiledMapTerrainContent`. Terrains and the newer WangSets are only used in constructing a map, not for rendering. Added exception throws for non-implemented parts of the format.
Added the ability to load files with groups. Rendering system still doesn't understand layer groups yet, so this allows groups to be initially loaded, but the sub layers of the group are extracted to the layer root, things like offsets for the group are applied directly to the sub layers, and the groups themselves are removed.
`FirstGlobalIdentifier` is only used with the map to reference the tileset which contains the tile referenced. It isn't really part of the definition of a `TiledMapTileset` even though the `.TMX` format includes it with the `<tileset>` tag
Pulled out the methods for writing and reading the content files for tilesets into their own classes to allow loading tilesets independently from the maps.
Changed the reference of tileset textures so the `Texture2DContent` isn't referenced, but built into the file as tilesets don't reference the same texture more than once.
