Fix slider keyboard interaction.
Fix slider keyboard interaction.

- Now `Slider` can be itself focused, handle the key events from the control, not the `Track`.
- Reverse the direction in which the page up/down keys move the slider in order to work as expected
- Make up/down arrows work as well as left/right

Fixes #4205

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