QDockWidget : add support for detaching as regular windows

Sometimes one would like to be able to detach the view provided by a QDockWidget and have it behave like a normal, standalone window instead of as a tool window.

• regular windows stay visible when the application focus changes
• regular windows can do stacking and can thus go (partly) under the main window
• regular windows can be put (partly) off the screen

A good example candidate would be an embedded documentation browser.

It turns out to be very tricky to change the detached windows externally, but a quick test suggests that it's very simple in the private QDockWidget method that does the actual state change. Just set `Qt::Window|Qt::WindowMinMaxButtonsHint` instead of `Qt::Tool`.

I'd like to propose to make this behaviour optional by providing an additional flag along the lines

void setFloatsAsStandalone(bool standalone);
bool floatsAsStandalone();

or

Q_PROPERTY(bool floatStandalone READ floatsAsStandalone WRITE setFloatsAsStandalone) 

Let me know if you have a better suggestion for the name of the state variable and its access methods, and I'll propose a patch (against 5.9LTS if that's OK).