wmaker: Add new options for dragging maximized windows.

You can now configure the behavior when dragging a maximized window by setting
DragMaximizedWindow in ~/GNUstep/Defaults/WindowMaker.  The options are:
- Move: Move the window and retain its maximized status and geometry (the
  current behavior and the default).
- RestoreGeometry: Move the window and unmaximize it, restoring its original
  geometry.
- Unmaximize: Move the window and unmaximize it, retaining its maximized
  geometry.
- NoMove: Don't move the window.