Asks the `GtkTreeDragDest` to insert a row before the path dest, deriving the contents of the row from value.
If dest is outside the tree so that inserting before it is impossible, false will be
returned. Also, false may be returned if the new row is not created for some model-specific reason.
Should robustly handle a dest no longer found in the model!
| this |
a `GtkTreeDragDest` |
| dest |
row to drop in front of |
| value |
data to drop |
|
whether a new row was created before position |