This was accidentally commited by John and causes the triple-click bug.
I think the reason is, at least in some versions of GTK+, indicating
that we handled the double-click means that the double-click can't be
used for firing activation. In other versions of GTK+, it seems the
treeview looks at each GDK_BUTTON_PRESS separately, and does its own
double-click testing and will still work.
References #11942