ENH: give annotations option to be visible only in selected views

This uses the same convention used in dislay nodes for other
displayable managers (e.g. VolumeRendering), except that it uses the
sliceNode since it is the 2D analog to the 3D view Node (at some point
perhaps there should be a generic superclass). Here, since we are only
using the ID string it doesn't matter what type the node it.

This feature allows custom UIs, for example where some annotations only
show up in one view while others show up everywher. In particular, this
is helpful for annotations representing volume-specific anatomical
landmark locations.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21740 3bd1e089-480b-0410-8dfb-8563597acbee