Allow BindingMode.Default in Setter bindings.
Allow BindingMode.Default in Setter bindings.

Fixes #1754.

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