[WPF] Fix drag&drop issue
[WPF] Fix drag&drop issue

Do an additional DragOver check before dropping, since the Drop event may
come without a DragOver. Also properly initialize the initial drag area.

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