MonoGame Content Builder Fixes

A few random fixes for some issues i found tonight:

 - Fixed cases where "dot notation" was left in file paths (C:\paths\like\..\..\this\should\not.exist)
 - The project, output, and intermediate directory paths now always have a single trailing separator.
 - Fixed crash within SongProcessor if the output path for the song did not exist.
 - Fixed SongContentWriter to write duration with type information.