Rework HasSystemDecorations.
Rework HasSystemDecorations.

Make it a direct property that is backed by `SystemDecorations`. Moved the changed handlers to `OnPropertyChanged`.

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