Enable TreeView horizontal scrollbar. (#1872)
Enable TreeView horizontal scrollbar. (#1872)

Fixes #1871.
jkoritzinsky/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia,wieslawsoltes/Perspex,akrisiun/Perspex,Perspex/Perspex,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,wieslawsoltes/Perspex,SuperJMN/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,wieslawsoltes/Perspex,SuperJMN/Avalonia,grokys/Perspex,grokys/Perspex,AvaloniaUI/Avalonia,SuperJMN/Avalonia,wieslawsoltes/Perspex,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,wieslawsoltes/Perspex,AvaloniaUI/Avalonia,jkoritzinsky/Perspex,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,wieslawsoltes/Perspex,AvaloniaUI/Avalonia,Perspex/Perspex,wieslawsoltes/Perspex