[Xwt.Gtk] Use the LinkLabel widget properly in the MarkDown widget
[Xwt.Gtk] Use the LinkLabel widget properly in the MarkDown widget

Use the widget registry to correctly get the native widget from the
xwt widget.

antmicro/xwt,steffenWi/xwt,mminns/xwt,sevoku/xwt,iainx/xwt,akrisiun/xwt,hamekoz/xwt,TheBrainTech/xwt,cra0zy/xwt,hwthomas/xwt,mminns/xwt,lytico/xwt,residuum/xwt,mono/xwt,directhex/xwt