* gtk/gtktreeprivate.h: add fixed_height_check field
* gtk/gtktreeview.c (gtk_tree_view_init): initialize
scroll_sync_timer and fixed_height_check
(do_validate_rows): add fixed_height_check. If all validated rows
in the first cycle have the same height, then we set that height for
the entire tree. This is some sort of 'fake' optimization, but helps
a lot for the common case. We keep validating the entire tree in
the background though.
(gtk_tree_view_set_model): reset fixed_height_check