Fix some issues in the modelview example

The edit menu was never implemented and should not be exposed.
Fading out the combo box is not really intuitive and doesnt actually
disable the control.

task-number: QTBUG-33818
Change-Id: I76190b675fd9400caac2754277ba9960b7f6dd06
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Menu: Remove reported warning

Reported as a comment here,
https://bugreports.qt-project.org/browse/QTBUG-31568

Change-Id: I84df10a584d09bd25359b9120d5e219bdf1bf9d3
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>