These icons will be used in the upcoming new implementation of the print
preview frame and as they are also natively supported by GTK+ it makes sense
to have support for them in wx itself.
Notice that the existing bookmark add/remove icons are already mapped to the
icons which look like plus and minus signs respectively in wxGTK but we need
plus/minus in print preview in the other ports too so add these icons under
explicit names.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66504 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
