* .gitmodules: Add MonoGame-SDL2 submodule.

* LDEngineContent.Linux.contentproj: New content project for Linux
  builds.

* Silkscreen.ttf: MonoGame content projects on Linux insist that the
font file be in the local directory with the appropriate
name.

* blank.png:
* testhero.png:
* particles.png:
* testtiles.png:
* font.spritefont: MonoDevelop doesn't like these as links to the
  other content
project. It attempts to link to versions within the build directory.

* .gitignore: Ignore IDE file.
File not needed here.