Kill gtk_tree_view_size_request

Kill gtk_tree_view_size_request

Splits up size_request() so that the height calculations are only done
when get_preferred_height() is called and the width calculations are
only done when get_preferred_width() is called.  Since
get_preferred_width() does not change the treeview->priv->width value,
treeview->priv->prev_width will always be equal to it and can therefore
be removed.  The only place where prev_width was used is a block in
gtk_tree_view_size_allocate().  This block seems to be adjusting the
horizontal scrollbar to account for treeview->priv->width having been
changed in size_request() and should no longer be necessary.  A similar
block immediately above it seems to already account for the width change
in size_allocate().

https://bugzilla.gnome.org/show_bug.cgi?id=691751