Designer: Extend container extension.

Designer: Extend container extension.

Make it possible to disable adding/removing pages by
adding respective bool virtual functions.

Useful for implementing containers with fixed, single
children like QScrollArea, QDockWidget, which require
a container extension to work properly in Qt Designer.
Previously, the problem was that the add/remove
page context menu actions were enabled for them,
leading to crashes und undesired behaviour.

See e00b8105bfcf3d4e36275dea6355705ec5fdbc40 in qtbase.

Reviewed-by: Jarek Kobus <jaroslaw.kobus@nokia.com>