SharedDialog.view.MainContainerController: enhance the demo to support gaps between windows

example: a user could manually drag a docked window away from the main window or apps could position windows initially not right next to each other.

Obviously, there is no visible drag proxy in place when dragging not over browser windows.

To implement this onDragStart() needs to call Neo.Main.getWindowData() on all connected windows. The returning data object contains e.g. screenLeft, screenTop, otherHeight, outerWidth => calculating gaps is fairly easy.

Adding the label "good first issue", since it does not require deeper knowledge of the framework.

Adding the label "help wanted", since it is not a high priority item (unless many users ask for it).

A lot of other tasks on my plate.