(ui): improve gui aesthetic
What does this MR do?
- Improve the background management in 'fwGuiQt'.
- Disable parameters in 'uiMeasurement::SLandmarks' instead of hide them.
- Allow to configure the style of 'SActivityWizard'.
- Poperly use the 'hideActions' mode on toolbars.
- Properly set bounding boxes on 'visuOgreAdaptor::SAxis'.
- Add a flatdark theme.
How to test it?
Test Landmarks in VRREnder