Improve RangeSlider

[timunie]

Descrition of changes

• Use CoerceValue where possible (same as Slider in Avalonia uses)
• Add property to control snap behavior IsSnapToStepFrequencyEnabled (not everyone may like it, same as Slider in Avalonia)
• SnapToFrequency should be handled in input events, not in PropertyChanged (don't touch Values from the ViewModel sent)

To test the RangeSlider

I added a minimum sample to SandBox. Can revert this change before merging if you want me to

Tests

Well ... Need to think about useful tests later on, if that PR gets accepted.

Fixed issues

Fixes #568

[timunie]

Hi @amwx

since 2.1.0 seems to be released already, should I switch target branch?

[dowdybrown]

We use the RangeSlider in our project and can't update to Avalonia 11.1.x until this pull request is approved and released. I would be happy to provide any assistance needed toward that end.

[timunie]

@dowdybrown found out that the underlaying issue is solved in latest Avalonia nightly. However FA needs some updates to handle this version so we will need to wait for a fix or build FA on our own.