Global response handlers

Added

• Added global error handling capability. Read more here.
• Added easy task retry. Read more here.
• Exposed access token in clients.
• Added isRequiredDropboxAppInstalled for OpenWith partners.

Fixed

• Improved token migration handler behavior.
• Separate clients by auth-type, not namespace.
• Fixed app-extension safety regression.
• Improved batch upload to handle very large number of files.
• Fixed redefining block variables, and added explicit self-> references.