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

Port change from r21740 from Annotations to Markups

    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

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22682 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: clarify name of attribute (Markup not Seed)

There are different conventions in different parts of the code,
but Seed is consistently the vtk widget layer, and Markup is the
Slicer nomenclature.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22683 3bd1e089-480b-0410-8dfb-8563597acbee