Update submodules: plugins (new commits)

plugins:
    c6ee754 Chris Smith <chris@dmdirc.com>
            Pass an IconManager into StandardInputDialog.

            Removes some deprecated method calls.

            Gerrit: http://gerrit.dmdirc.com/3090

Change-Id: I40138ada1997a7f807aaa7e0a3cdf6b06ed471c0