clutter: Move Image down to GNOME Shell

As is, ClutterImage is not really useful, it only serves for rendering a CoglTexture as an ImageContent. Shell, has a subclass that adds more features that unfortunately cannot be upstreamed without bringing more gdk-pixbuf usage inside libmutter, eg implementing GIcon/GLoadableIcon.

It also has requirements based on whether the image is symbolic or not. Things that Clutter so far doesn't care about.

So just remove ClutterImage & let shells re-implement it themselves if needed based on their needs.

Note, that once we have ClutterSnapshot, it should be straightforward to write a custom actor that renders a CoglTexture or so.

This "un"fortunately means getting rid of various interactive tests that either didn't compile at all or are not useful as is, like all the remaining interactive tests.