QDockWidget: Improve <CTRL> docking prevention support

What happens today (Qt 5.12.6 + 5.14):
When you click-drag a floating QDockWidget around any QMainWindow for the app, it will attempt to dock the widget as you float over the different DockWidgetAreas of the main window (it will resize widgets in the main window, showing a placeholder where your dockwidget would dock to if you release the mouse button).

To avoid this behavior today, you can press and hold <CTRL> BEFORE you click to drag the position of the floating QDockWidget. The result is that you can reposition the QDockWidget floating window without affecting the QMainWindow - the QMainWindow will not resize it's contents to show the different DockWidgetAreas, and if you release the window anywhere, it will not dock with any QMainWindow.

This option is cumbersome as you must remember to press <CTRL> prior to dragging the floating QDockWindow around.

Proposed new behavior:

Improved behavior would be for the user to be able to press the <CTRL> modifier at any time during the drag of the floating QDockWidget window to prevent or allow the docking behavior.