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/include/sixtyfps.h

@@ -670,6 +670,11 @@ void run_event_loop()
     cbindgen_private::sixtyfps_run_event_loop();
 }
 
+void quit_event_loop()
+{
+    cbindgen_private::sixtyfps_quit_event_loop();
+}
+
 /// Registers a font by the specified path. The path must refer to an existing
 /// TrueType font font.
 /// \returns an empty optional on success, otherwise an error string