[Xwt] Fix one strong event handler registration on strong ref cycle
[Xwt] Fix one strong event handler registration on strong ref cycle

There still is one created by EnableEvent which I'm not sure how to fix
yet

TheBrainTech/xwt,cra0zy/xwt,hamekoz/xwt,lytico/xwt,antmicro/xwt,mono/xwt,hwthomas/xwt