Update submodules: plugins (new commits)

plugins:
    8e3fa1c Chris Smith <chris@dmdirc.com>
            Remove windowClosing() methods from plugins.

            Override the close() method instead as per earlier core change.

            Gerrit: http://gerrit.dmdirc.com/3012

Change-Id: Ia3d2bb20c542faf1f37892f4bb921c70fdd849b2