recorder: rename "filename" property to "file-template"

The filename property is actually a template string with
substitution variables, not a filename.

This commit renames for clarity.

https://bugzilla.gnome.org/show_bug.cgi?id=680647

src/test-recorder.c

@@ -46,7 +46,7 @@ on_stage_realized (ClutterActor *stage,
                    gpointer      data)
 {
   recorder = shell_recorder_new (CLUTTER_STAGE (stage));
-  shell_recorder_set_filename (recorder, "test-recorder.ogg");
+  shell_recorder_set_file_template (recorder, "test-recorder.ogg");
   shell_recorder_record (recorder);
 }