MGCB Support For Content Projects

This PR adds support for using the MonoGame content builder tools with an existing XNA .contentproj without any need for XNA Game Studio to be installed on the system.

 - Added fix to MGCB.exe avoid exception when content listing file doesn't exist.
 - Added /quiet switch to MGCB.exe.
 - Added new .targets file which replaces the `Microsoft.Xna.GameStudio.ContentPipeline.targets` import.

The usage is fairly simple:

 1. If you are using custom content processors rebuild them against MonoGame.
 2. Replace the <Import> of `Microsoft.Xna.GameStudio.ContentPipeline.targets` with `MonoGame.Content.Pipeline.targets`.
 3. Place MGCB.exe and its dependent assemblies into the `$(MSBuildExtensionsPath)\MonoGame\v3.0\` folder.
 4. Build.

Please give it a test and report back any issues.