Copy URI to XA_CLIPBOARD on right click

Small patch that copies the match into the default clipboard using
gtk_clipboard_set_text(). Maybe consider which clipboard to use
(such as PRIMARY OR SECONDARY) instead based on a config option, although
that may be overkill (especially since PRIMARY is supposed to be ephemeral
or something)

Signed-off-by: Simon Gomizelj <simongmzlj@gmail.com>