Scrolls the tree view such that the top-left corner of the visible area is tree_x, tree_y, where tree_x
and tree_y are specified in tree coordinates.
The this must be realized before this function is called. If it isn't, you probably want to be using scroll_to_cell.
If either tree_x or tree_y are -1, then that direction isn’t scrolled.
| this |
a `GtkTreeView` |
| tree_x |
X coordinate of new top-left pixel of visible area, or -1 |
| tree_y |
Y coordinate of new top-left pixel of visible area, or -1 |