Even More Pipeline Fixes 

 
 - Fixes to UI abstraction in Pipeline GUI tool.
 - We now list all content importers instead of limiting them by file type.
 - We now define default output and intermediate folders for content projects.
 - Paths are wrapped in quotes when saving content projects.
 - Implemented `ContentProcessorContext.BuildAndLoadAsset()`.
 - We now properly clean extra output files like songs.
 - Fixed project saving in Pipeline GUI tool.

Fixes #1682.