Add HighContrast icons

The svg source (HighContrast_source_ghex.svg) is the original creation
of wadhwa.sanchi@gmail.com. The HighContrast png renderings were done
with the create-highcontrast utility from gnome-themes-standard by
Matthias Clasen.

https://bugzilla.gnome.org/show_bug.cgi?id=687313