ENH: #1930 Don't scroll past edges of volumes

Now the arrow keys and mouse wheel events are clamped to the range
of the volumes in the z direction of the slice views like the view
controller's sliders are.  This prevents 'runaway' scrolling and losing
the slice.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22157 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: setting inputs to NULL when selectors not initialized; attempt to fix 3117


git-svn-id: http://svn.slicer.org/Slicer4/trunk@22158 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Add depth peeling option in 3D view controller

Closes #1798

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22159 3bd1e089-480b-0410-8dfb-8563597acbee
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