A picture is worth a thousand words:
Since a few hours ago both WebKit GTK and Qt support plugins! Thanks to everyone who worked on this, in particular to Rodney Dawes (who wrote the original patch in the past months) and to Marc Ordinas i Llopis (who maintained the patch and worked on the Qt port, and who recently joined us at Collabora). A thank you also to Alp Toker and Simon Hausmann who reviewed the patch.
Update: See also Marc’s blog for details.