When passing as 'anchor' a Geom object (ie. a widget dimen
or a ges.pos), or a function returning such an object,
a MovableContainer will be initially positionned near this
point/widget, instead of being centered on the screen.
Allow that for ButtonDialog and ButtonDialogTitle, so we
can make them behave as context menus (ie. for a titlebar
top left/right icons).
8036ca0d56