property_tracker: don't register dependencies for tracker that don't …

[ogoffart]

…have dependencies themselves

This saves 3% of dependencies in my benchmark

[ogoffart]

I'm not 100% this is correct because the PropertyTracker has a set_dirty function.
The use in windows.rs should be fine because the window_properties_tracker isn't itself used as a dependency. But the accesskit one needs double checking

[tronical]

That’s very tempting. I’m also not 100% sure. What if the tracker hasn’t done its initial evaluation but will do so later on?