desktopGrid,Manager: Use dedicated clipboard mime type
Cut/copy/paste has originally been implemented (ab)using the plain text clipboard because that's the only data type St.Clipboard supported.
This results in implementation details leaking into the text clipboard, and has required nautilus to regress in order to interoperate with us.
But now St.Clipboard can handle any type of clipboard content, so nautilus has restored its clipboard mime type and we shall port to it.
Needed for GNOME/nautilus#634 (closed)