Update submodules: plugins (new commits)

plugins:
    31dc4b5 Chris Smith <chris@dmdirc.com>
            Use services for frame managers.

            To add a frame manager, a plugin must export a FrameManagerProvider instance.
            This will drop users back to using a treeview as their previous config setting
            will be wrong (it needs to point to the Provider class now), and I don't think
            it's worth adding migration code given we know of 0 users of the buttonbar.

            This also enables us to dependency inject the frame managers.

            Gerrit: http://gerrit.dmdirc.com/3283

Change-Id: I4e70428f7941bb0cb3c01f300723ef8b3da75e24