Pipeline Tool Improvements

More improvements to the Pipeline GUI tool:

 - You can now toggle "Debug Mode" in the build menu and it will prompt the launch of the VS debugger for debugging the content building process.
 - Fixed error where the build/rebuild items were popup menus.
 - Deletion of items no longer leaves folders without children.
 - The context menu for the tree view is now common for folders and items.
 - Rebuild in the tree context menu now works when you multiselect items.
 - Fixed the "Add Items..." context menu handler.