Fix recurring C++ timers

* sixtyfps_timer_start needs to *take* the timer id out of the Rust
  timer to avoid that the subsequent drop stops the timer again
* For the Qt event loop, call `timer_event()` once before entering
  QCoreApplication::exec(), to schedule any timers that were started
  beforehand.
* Added a way to quit the event loop gently, in order to use that
  from the C++ unit test.

api/sixtyfps-cpp/lib.rs

@@ -37,6 +37,11 @@ pub unsafe extern "C" fn sixtyfps_run_event_loop() {
     crate::backend().run_event_loop();
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn sixtyfps_quit_event_loop() {
+    crate::backend().quit_event_loop();
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn sixtyfps_register_font_from_path(
     path: &sixtyfps_corelib::SharedString,