theme: Add high-contrast variant

For now, simply enforce symbolic icons.

https://bugzilla.gnome.org/show_bug.cgi?id=740447

data/gnome-shell-theme.gresource.xml

@@ -16,6 +16,7 @@
     <file>filter-selected-ltr.svg</file>
     <file>filter-selected-rtl.svg</file>
     <file>gnome-shell.css</file>
+    <file>gnome-shell-high-contrast.css</file>
     <file>logged-in-indicator.svg</file>
     <file>message-tray-background.png</file>
     <file>more-results.svg</file>

data/theme/gnome-shell-high-contrast.css

@@ -0,0 +1,20 @@
+/* Copyright 2014, Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU Lesser General Public License,
+ * version 2.1, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St - Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+@import url("resource:///org/gnome/shell/theme/gnome-shell.css");
+
+stage {
+  -st-icon-style: symbolic;
+}