Don't show TextBox caret when control not focused.
Don't show TextBox caret when control not focused.

Fixes #836.

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