Refactored styling.
Refactored styling.

- Don't use Rx in the styling system. Instead introduces `IStyleActivator` which is like an `IObservable<bool>`-lite in order to cut down on allocations.
- #nullable enable on touched files

Perspex/Perspex,SuperJMN/Avalonia,wieslawsoltes/Perspex,SuperJMN/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,wieslawsoltes/Perspex,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,wieslawsoltes/Perspex,akrisiun/Perspex,jkoritzinsky/Perspex,AvaloniaUI/Avalonia,wieslawsoltes/Perspex,SuperJMN/Avalonia,jkoritzinsky/Avalonia,wieslawsoltes/Perspex,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,Perspex/Perspex,SuperJMN/Avalonia,grokys/Perspex,wieslawsoltes/Perspex,wieslawsoltes/Perspex,grokys/Perspex,jkoritzinsky/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia