Move those two classes into the private_api namespace, which is excluded
from the API reference documentation.
For generate code the explicit qualification of Property<T> is changed,
for the cbindgen generated item types the private_api namespace is
pulled into the cbindgen_private namespace.
* 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.
By default PropertyTracker::evaluate() registers the currently
evaluating binding/tracker as a dependency. This should help with
repeaters and other scenarios where in the run-time we use property
trackers but want to track the overall "dirtyness" in the window with
regards to whether a redraw is needed or not.
The new evaluate_as_dependency_root() function allows skipping this
mechanism and is used for the two trackers in the window.
