New Application class, removed PyMEL

• New Application class for testing current application name, version, batch mode, etc
• Fix generic deferred method not passing in arguments
• [Maya] Replaced pymel with maya.cmds
• [Maya] Fixed error loading dialog windows
• [Maya] Fixed bug where windows wouldn't dock via code
• [Nuke] Fixed error loading standalone and terminal windows
• [Nuke] Show traceback if error occurs during callback
• [Blender] Add deferred method