ENH: #1879 hotkeys to cycle through volumes

Added left/right bracked [ and ] to cycle through background layer
and left/right brace { and } to cycle through foreground layer.

Label layer is not included since labels can be seen in foreground
layer.

Also remove some trailing spaces for sytle purposes.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22160 3bd1e089-480b-0410-8dfb-8563597acbee