Retrieves the selection bound of the editable.
start_pos will be filled with the start of the selection and end_pos with end. If no text was selected both
will be identical and false will be returned.
Note that positions are specified in characters, not bytes.
| this |
a `GtkEditable` |
| start_pos |
location to store the starting position |
| end_pos |
location to store the end position |
|
true if there is a non-empty selection, false otherwise |