fileUtils: Add helper for loading D-Bus XML from resource

Commit dbf993300a moved all inline D-Bus interface descriptions to template
strings so we can stop escaping line breaks.

Unfortunately that unveiled a grave bug in xgettext, which currently cannot
handle files that contain both backtick and slash characters - as a result,
translations from affected files have started to disappear as translators
run xgettext/msgmerge.

Instead of reverting the change and getting the crusty escaping back, we
will take this as an opportunity to stop inlining the XML altogether and
load it from a resource instead.

To facilitate that, add a small helper method that loads a D-Bus interface
description from a dedicated resource bundle.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/537

js/misc/fileUtils.js

@@ -3,6 +3,7 @@
 const Gio = imports.gi.Gio;
 const GLib = imports.gi.GLib;
 const Lang = imports.lang;
+const Config = imports.misc.config;
 const Params = imports.misc.params;
 
 function collectFromDatadirs(subdir, includeUserDir, processFile) {
@@ -70,3 +71,29 @@ function recursivelyMoveDir(srcDir, destDir) {
             recursivelyMoveDir(srcChild, destChild);
     }
 }
+
+let _ifaceResource = null;
+function loadInterfaceXML(iface) {
+    if (!_ifaceResource) {
+        // don't use global.datadir so the method is usable from tools
+        let path = Config.PKGDATADIR + '/gnome-shell-dbus-interfaces.gresource';
+        _ifaceResource = Gio.Resource.load(path);
+        _ifaceResource._register();
+    }
+
+    let xml = null;
+    let uri = 'resource:///org/gnome/shell/dbus-interfaces/' + iface + '.xml';
+    let f = Gio.File.new_for_uri(uri);
+
+    try {
+        let [ok, bytes] = f.load_contents(null);
+        if (bytes instanceof Uint8Array)
+            xml = imports.byteArray.toString(bytes)
+        else
+            xml = bytes;
+    } catch (e) {
+        log('Failed to load D-Bus interface ' + iface);
+    }
+
+    return xml;
+}