Add Linux solution files.

* map.tmx: Add a version of map.tmx file for Linux. This can be
  unified with the Windows file, after push of updated example
  game code.

* LDEngine.Linux.csproj: and TiledLib.Linux.csproj: Add
  Linux-specific project (to allow for different references).

* LDEngineContent.Linux.contentproj: Content project for Linux.
  This is required to build the spritefont. It is not necessary to
  compile the other PNG files, as MonoGame can use them
  directly - they are only included for completeness.

* Silkscreen.ttf: Add font file for Linux.
  With the current Linux pipeline, the font must be in the
  same directory and named to match the tag in .spritefont.

* blank.png, testhero.png, particles.png, testtiles.png:
  Add files for Linux project.  Linking to Windows content
  project files will not work, due to the incomplete
  content pipeline implementation in the current
  MonoGame.

* font.spritefont: SpriteFont file for Linux.

* .gitignore: Add ignore for MonoDevelop user preferences.

* Program.cs: Removed conditionals that limited the program
  loop to Windows/360 only.
Formatting Updates

* Map.cs: Convert tabbed lines to spaces to match
  project format.

* TiledLib.Linux.csproj: Set project preferences to
  match VisualStudio spacing/formatting.
Added raw TMX Loader.

* ExampleGameplayScreen.cs: Update 
  ExampleGameplayScreen to call new  constructor.
  ToDo: add fallback to XNB if .tmx not present?

* Map.cs: Add map constructor that can read a .tmx file.
  Updated old XNB constructor to build tiles compatible
  with collision changes.

* PropertyCollection.cs: Added
  PropertyCollection.Add(KeyValuePair<string, string>).

* TileLayer.cs: Add TileLayer constructor that reads a TmxLayerTile
  List.