Revert "clipboard: Use text based clipboard only"

After removing the non-default "icons on desktop" functionality from nautilus, a gnome-shell extension was created as a substitute. This extension needed to interoperate with nautilus for file operations, but its access to clipboard was limited to plain text, so, in order to ensure interoperation, nautilus had to use plain text clipboard only.

However, this temporary regression resulted in implementation details leaking into the text clipboard, instead of a plain file path.

But now St.Clipboard can handle any type of clipboard content[1], the extension will be able to handle any clipboard type, so we can finally fix this regression on our side.

This reverts commit 1f77023b and its fixup commit be53569b

Fixes #634 (closed)