clipboard-manager: Get images from GtkClipoard

Fetching images via GtkClipboard seems to work correctly, as did the old version of clipboard manager used in clipman (see #105 (closed)). On the other hand, fetching text can cause applications to hang on closing (e.g. mousepad, see panel-plugins/xfce4-clipman-plugin#107 (closed)), whereas it works correctly via X11 code.

So here we try to get the best of both: if an image is available, we use GtkClipboard, ignoring X11 events, and do the opposite for the rest.

Fixes: #105 (closed) Related: panel-plugins/xfce4-clipman-plugin#107 (closed)