Fix possible double-paste bug

Fixed a possible bug where both text and files would be sent if the
owner of the selection allowed for both in xlib.
Add support for copy/pasting inline images in Xlib

Images can be pasted from the clipboard if the owner can provide them in
an image/png format.  No support for incremental pasting yet so large
images do not work.  There seems to be a problem with RGB/BGR encoding
but it only affects the preview, and shows up fine in other clients
(tested with venom).
Handle escaped special characters in filenames

uTox now properly supports percent-escaped URIs from dragging and
dropping and from pasting.
Merge branch 'master' of https://github.com/blucoat/uTox