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/popupMenu.js

@@ -252,7 +252,7 @@ PopupImageMenuItem.prototype = {
         if (!show) {
             this._imageBin.hide();
         } else {
-            let img = St.TextureCache.get_default().load_icon_name(this._iconName, this._size);
+            let img = St.TextureCache.get_default().load_icon_name(this._iconName, St.IconType.SYMBOLIC, this._size);
             this._imageBin.set_child(img);
             this._imageBin.show();
         }