At the moment tooltips can be set for any widget with a window. However,
these tooltips are static.

If there was the ability to set a callback which is called on a particular
widget with x,y co-ordinates when a tooltip is about to be displayed, the
function could do arbitary work (i.e. retrieve data from a GtkTreeModel) so
I could have tooltips for cells in TreeViews.

If this is implemented, please don't make the Java Swing mistake of calling
the callback every time the mouse moves, even if no tooltip is about to be
displayed... :)