Add symbolic icons to TextureCache's load_icon_name()

Icons can be loaded as St.Icon.SYMBOLIC, FULLCOLOR, APPLICATION or
DOCUMENT. The first will look for a symbolic equivalent, the second
looks for a full-color version (and does fallback, eg, from
"drive-harddisk-usb" to "drive-harddisk"). APPLICATION and DOCUMENT do
full-color icons without fallback (as specified by the icon spec).

And update various callers to use the right flags.

Based on a patch from Matt Novenstern.

https://bugzilla.gnome.org/show_bug.cgi?id=621311

js/ui/telepathyClient.js

@@ -347,7 +347,7 @@ ContactManager.prototype = {
             let uri = GLib.filename_to_uri(file, null);
             iconBox.child = textureCache.load_uri_async(uri, iconBox._size, iconBox._size);
         } else {
-            iconBox.child = textureCache.load_icon_name('stock_person', iconBox._size);
+            iconBox.child = textureCache.load_icon_name('stock_person', St.IconType.FULLCOLOR, iconBox._size);
         }
     },