Make it obvious that adding a QLayout to QSplitter is not supported.

Make it obvious that adding a QLayout to QSplitter is not supported.

It does not make sense to add a QLayout to a QSplitter, since the
splitter manages its child widgets in the same manner as a QLayout.
The result of doing so is that the child widgets inside that layout
will lead to the splitter and the layout fighting to position the child
widgets.

QSplitter::addWidget should be used to add widgets directly to the
splitter instead.

Change-Id: I640b463cae8673f87354d28636bff4dd3cfb9679
Reviewed-by: Samu Voutilainen <samu.voutilainen@gmail.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>