Qt5 misc fixes
Created by: vovythevov
This PR addresses the following issues for the Qt5 transition:
- In Volume Rending module, the items of the ‘View’ combobox are no longer checkable.
- Styles are broken (cannot select Slicer Dark Style)
- QComboBox becomes empty after selection changed (https://issues.slicer.org/view.php?id=4456)
Note that the base style has changed to QFusionStyle. This creates slight differences in the Slicer's appearance. As an example, the landing page in fusion (Qt5):
And with Qt4: