Can be tested with kddockwidgets_example -c
Disabled by default, as I'm not 100% happy with it:
A dock widget with the "non closable" attribute will still show
the x button on the tab bar. Clicking on it won't do anything but
doesn't look so nice. Qt doesn't allow to have the close buttons only
in some tabs.
Fixes#26
By default a dock widget can dock into any main window.
With affinities, we can now have a dock widget "belong" to a main window
and only be able to dock into it (or into other floating dock widgets
with the same affinity).
See DockWidgetBase::setAffinity() and MainWindowBase::setAffinity().
