Allow injection of a global IconManager.

Change-Id: I0a6ccc960a4e08c0a7c6afc08a75b89d2b17ccae
Reviewed-on: http://gerrit.dmdirc.com/3014
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Update submodules: plugins (new commits)

plugins:
    20c1644 Chris Smith <chris@dmdirc.com>
            Use injected IconManager

            Gerrit: http://gerrit.dmdirc.com/3015

Change-Id: I97f21984d738ed769513d5f5bd650cc03e8cedd8