PipelineTool, Import XNA project

Can now import an XNA .contentproj using File > Import.

 - Microsoft assemblies referenced by the contentproj are ignored.
 - MonoGameContentProcessors assembly referenced by the contentproj are ignored.
 - Processors / Importers which could not be found will appear as "Invalid / Missing" in the property grid.
 - Editing the project references will trigger a reevaluation of all importers / processors to locate 'missing' ones.

If your game was previously using MonoGameContentProcessors.dll, all those processors are now defined in Monogame.Framework.Pipeline.dll (which does not need to be explicitly added as a reference). PipelineTool will automatically remap to the new name for certain processors (such as MGTextureProcessor -> TextureProcessor).

Note that your game's pipeline assembly which contains custom processor/importers must be built against Monogame.Framework.Pipeline (and not just that but the same version of it that the PipelineTool is built against) or the PipelineTool will fail to extract its Processor / Importer types.