Remove WritableFrameContainer.

Move the functionality down into just FrameContainer. Callers can use
isWritable() to determine if the FrameContainer is writable, instead
of horrible instanceof/casting logic.

Change-Id: Iab58f1d2b9275ee3fc60d79e433e158bae784911
Depends-On: I52a09d5f5ad69c2bebafa58582a173f706515fa6
Reviewed-on: http://gerrit.dmdirc.com/3426
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Update submodules: plugins (new commits)

plugins:
    308c21b Chris Smith <chris@dmdirc.com>
            Remove WritableFrameContainer.

            Depends-On: Iab58f1d2b9275ee3fc60d79e433e158bae784911
            Gerrit: http://gerrit.dmdirc.com/3425

Change-Id: Ic752da7665e041d9b9bc68a0bd9ac525544a0efd