[SegmentedControl] Adding AutoSelection and high contrast improvements

[niels9001]

See #392

This PR introduces the following changes:

• Adds the AutoSelection property (by default the first item will be selected - this can be disabled if needed)
  

• High contrast improvements
  

[michael-hawker]

Ugh, silly GitHub requires a comment here if you do things out of order... 😋

[michael-hawker]

@niels9001 FYI, I noticed you had some usages of as with pragmas to ignore warnings of the null deref. Easier/safer pattern is to use the is typing like so:

        if (element is SegmentedItem segmentedItem)
        {
            segmentedItem.Loaded += SegmentedItem_Loaded;
        }

[michael-hawker]

FYI @niels9001 I noticed at different DPI (150?) the padding (gaps) is off:

[michael-hawker]

Same C# is cleanup can be done here:

            // Only do stuff if the index is actually changing
            if (index != previousIndex && ContainerFromIndex(index) is SegmentedItem newItem)
            {
                newItem.Focus(FocusState.Keyboard);
                retVal = true;
            }
        }

        return retVal;
    }

    private SegmentedItem? GetCurrentContainerItem()
    {
        if (ControlHelpers.IsXamlRootAvailable && XamlRoot != null)
        {
            return FocusManager.GetFocusedElement(XamlRoot) as SegmentedItem;
        }
        else
        {
            return FocusManager.GetFocusedElement() as SegmentedItem;
        }
    }

[michael-hawker]

@niels9001 was looking into the selection issue with SelectedIndex. I think the issue is that SelectedIndex/Item is read first before the Items collection has loaded, so there's a timing issue. (Also, through XAML the items load one-by-one I think.)

Was investigating if we could simplify the logic here than trying to check everytime an item is loaded if we've set the index. Will think about it more Monday.

(Though I'm also still surprised we have to do anything special here. I mean ListViewBase should just be handling all this logic for us.)

[michael-hawker]

Tested on UWP and WinAppSDK, looks great.

[michael-hawker]

@Arlodotexe feel free to take a quick poke if you'd like and/or just merge.