Update submodules: plugins (new commits)

plugins:
    894fb42 Chris Smith <chris@dmdirc.com>
            Tidy up topic handling.

            Depends-On: I9e8723fe0fe2b9209a401d33361f80fb832b9434
            Gerrit: http://gerrit.dmdirc.com/3414

    aede63c Greg Holmes <greg@dmdirc.com>
            Remove PreferencesManager singleton.

            Gerrit: http://gerrit.dmdirc.com/3413

Change-Id: I679159f03dafa4636bb658b8d239300e860fea53
Tidy up topic handling.

Instead of returning either null or a topic, use Optional<>.

Change-Id: I9e8723fe0fe2b9209a401d33361f80fb832b9434
Depends-On: If0a0609ab778df22c02b8dc1d349da45b4b5b16f
Reviewed-on: http://gerrit.dmdirc.com/3415
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Remove PreferencesManager singleton.

Change-Id: I6bc0a35ec29cbfc21b6ea2b45ea55849b0a8752a
Depends-On: I5fc194790dd1459d3dab546f306768d5b620626d
Reviewed-on: http://gerrit.dmdirc.com/3416
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>