Pass TabCompleter into base class.

There's no sane reason for this to be an abstract method - instead
just create the tab completer and pass it up to WritableFrameContainer.

Change-Id: Ifa80de021c22d2e302d4b66514923cd08c91cac4
Depends-On: I15ab9677fdbcb510ab87a69a07ce8ecb3a994f7f
Reviewed-on: http://gerrit.dmdirc.com/3422
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Update submodules: plugins (new commits)

plugins:
    48daeb0 Chris Smith <chris@dmdirc.com>
            Update plugins for core FrameContainer changes.

            Depends-On: Ifa80de021c22d2e302d4b66514923cd08c91cac4
            Gerrit: http://gerrit.dmdirc.com/3421

Change-Id: Ife2c6130f807785066cd2882899838f8ba800052