Deprecate windowClosed and windowClosing methods.

These are a hold-over from when the core had to have a back-and-forth with
the UI in order to close a window.

Provide stub implementations in FrameContainer, deprecate them, and remove
any unecessary overrides in the core.

Change-Id: Icf9bfc6bc40b66135d161a5047d01d46b2171234
Reviewed-on: http://gerrit.dmdirc.com/3008
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>