Add Direct TMX Loading Support

Hopefully this shouldn't introduce any new major issues.  A large chunk of it is just setting up the new project/solution code for Linux to get it in parity with Windows.

I didn't touch the Windows project, as I've been working solely from Linux.  The Windows TiledLib project file will need to be updated to add a reference for TiledSharp, and to copy the .TMX and its related .PNG to the output/Content directory, but other than that it should be unaffected (Optionally, the examplegamescreen could be updated to continue calling the old map load #if Windows).

The Windows build could be left to use the existing MonoGame references, or it could be updated to use MonoGame-SDL2 from the project directory (You'd need to run the submodule init & update commands inside of the MonoGame-SDL2 folder to pull down its own ThirdParty dependencies).  It's your project, so you can decide what works best.  :)

More details are in the descriptions of the individual commits.  We can talk in the Issue #1 comments, or via Twitter for any other questions/concerns.  Goodnight!