Don't focus Win32 window in design mode.
Don't focus Win32 window in design mode.

Fixes #837.

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