[WPF] Fix Xwt.Label accessibility
[WPF] Fix Xwt.Label accessibility

Wpf's LabelBackend uses custom WpfLabel, which sets TextBlock as
a Label's Content, and also redirects the focus to that element. Hence
we should use WpfLabel.TextBlock as an a11y target in AccessibleBackend.

antmicro/xwt,mono/xwt,lytico/xwt