treeview: Lower the duration of validation iterations

treeview: Lower the duration of validation iterations

GTK_TREE_VIEW_TIME_MS_PER_IDLE is currently 30 milliseconds, meaning
that validate_rows will validate rows up until all the validations have
taken over 30 msecs. So it's likely to block redrawing via the clock
frame update mechanism, as that tops at 16.66 milliseconds per frame
(1/60th of a second).

Stop validating rows if we've spent more than 3/5 of our allotted budget
for inter-frame processing, so as to avoid blocking.

In the future, we would probably want to calculate how long we would
have left until the next frame, especially if higher priority idles
and timeouts have already consumed a portion of that allotted time.

https://bugzilla.gnome.org/show_bug.cgi?id=726871