Get the TextIter at which the completion for
When implemented, this information is used to position the completion window accordingly when a proposal is selected in the completion window.
proposal text inside the completion window is aligned on
If this function is not implemented, the word boundary is taken to position the completion window. See activate_proposal for an explanation on the word boundaries.
proposal is activated, the default handler uses
iter as the start of the word to replace. See
activate_proposal for more