[Gtk] adjust mouse event positions
[Gtk] adjust mouse event positions

the mouse coordinates in events emitted by
the Gtk.TreeView do not include the column headers
and have an offset to the frontend bounds.

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