vtkXOpenGLRenderWindow: Support borderless and coverable windows

There does not seem to be a modern way to make X windows borderless but also not force them to be always-on-top. This change uses old motif window manager hints to achieve this goal. While motif is ancient, it seems most window managers still support these hints, and will continue to do so as long as X11 is around.

This change adds a Coverable ivar to vtkRenderWindow with a default setter implementation that just prints a warning. The vtkXOpenGLRenderWindow class overrides the setter to make use of the motif window hints when creating the X window, allowing the window to fall behind other windows, as well as be brought back to the front with the taskbar item.