treeview: Use equal areas in the computation of drag dest row positions

treeview: Use equal areas in the computation of drag dest row positions

We used to divide the row in thirds vertically, and use the outer thirds for GTK_TREE_VIEW_DROP_BEFORE and AFTER, respectively.

Now we use *fourths*.  This is so that we get equal areas for these:

  GTK_TREE_VIEW_DROP_BEFORE
  GTK_TREE_VIEW_DROP_INTO_OR_BEFORE
  GTK_TREE_VIEW_DROP_INTO_OR_AFTER
  GTK_TREE_VIEW_DROP_AFTER

This makes hovering tree rows much more positive.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>