[Xwt.Gtk] Use GtkEngine.ReplaceChild when setting multiple time .Content on a custom widget
[Xwt.Gtk] Use GtkEngine.ReplaceChild when setting multiple time .Content on a custom widget

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