loginManager: Move UnlockDialog.isSupported() here

With fallback mode dropped, we can no longer rely on gnome-screensaver
to be installed, so we'll have cases where we are unable to lock the
screen. The user menu should not show the "Lock" item in this case,
but as UnlockDialog includes UserMenu, we cannot use the existing check
without creating a circular dependency; move the function to a more
generic place to fix.

https://bugzilla.gnome.org/show_bug.cgi?id=693403

js/misc/loginManager.js

@@ -75,6 +75,35 @@ function haveSystemd() {
     return GLib.access("/sys/fs/cgroup/systemd", 0) >= 0;
 }
 
+function versionCompare(required, reference) {
+    required = required.split('.');
+    reference = reference.split('.');
+
+    for (let i = 0; i < required.length; i++) {
+        if (required[i] != reference[i])
+            return required[i] < reference[i];
+    }
+
+    return true;
+}
+
+function canLock() {
+    try {
+        let params = GLib.Variant.new('(ss)', ['org.gnome.DisplayManager.Manager', 'Version']);
+        let result = Gio.DBus.system.call_sync('org.gnome.DisplayManager',
+                                               '/org/gnome/DisplayManager/Manager',
+                                               'org.freedesktop.DBus.Properties',
+                                               'Get', params, null,
+                                               Gio.DBusCallFlags.NONE,
+                                               -1, null);
+
+        let version = result.deep_unpack()[0].deep_unpack();
+        return versionCompare('3.5.91', version);
+    } catch(e) {
+        return false;
+    }
+}
+
 let _loginManager = null;
 
 /**