WPrefs.app: Add ability to set behavior when dragging a maximized window.

You can now set the behavior when dragging a maximized window, i.e., the
"DragMaximizedWindow" option from ~/GNUstep/Defaults/WindowMaker, from the
"Window Handling" tab of WPrefs.app.

Note that to make room for the pop-up button required to set this option, the
switch button to set the "OpenTransientOnOwnerWorkspace" option has been moved
to the "Expert User Preferences" tab and the "Edge Resistance" frame has been
made slightly smaller.
NEWS: Add note about dragging maximized windows.