Adapt gtk_tree_view_get_path_at_pos() and
gtk_tree_view_get_drag_dest_row() to just return FALSE when bin_window
is NULL and not hit a warning. Makes this case consistent with the tree
view not having a model. Documentation has been updated to clarify
this, unit test has been added.
Fixes bug 539377, based on a patch by Bjorn Lindqvist.
Includes a test case for bug 546005 to start with, logic provided by
Paul Pogonyshev and Bjorn Lindqvist. In the future, we should maybe
merge treeview-scrolling.c with this one to create one large monolithic
tree view tester.
