Remove WritableFrameContainer.

Change-Id: I52a09d5f5ad69c2bebafa58582a173f706515fa6
Depends-On: Iab58f1d2b9275ee3fc60d79e433e158bae784911
Reviewed-on: http://gerrit.dmdirc.com/3425
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>