SectionTitleWidget::mousePressEvent call 'grabMouse' to ensure the floaing widget can always follow the mouse after floated.
SectionTitleWidget::mouseMoveEvent check Qt::LeftButton.
* Set parent of title- and content-widget to associated container-widget
* inside FloatingWidget::takeContent(...)
Restore state
* Restore all contents as visible and then call hideSectionContent(...) at
* the end. This is more consistent and uses same code (public API).
* Pass version to sub-methods to handle different formats
* Fix serialization, if nothing is visible (no sections, only hidden
* contents)
