Set cursor position from on_autocomplete callback

Nobody has claimed this yet.

on_autocomplete currently passes in the current cursor position, in order to allow autocomplete within a word/sentence at a specific point. This is nice, but not as useful as it could be, because the cursor will end up at the end of the autocompletion, which is the end of the entire input.

We need a way to set the cursor's position generally, like `c.set_cursor_position()`, but it also has to be understood by the caller of on_autocomplete whether or not this has been called. If it was called, dont put the cursor at the end, and if it wasn't, do.

Start by tracing the on_autocomplete callback and the code that currently places the cursor at the end of the completed input. Review the existing cursor API, including the proposed c.set_cursor_position() behavior, and determine how the caller can distinguish an explicitly set position from the default. Done means autocomplete preserves an explicitly selected position and otherwise retains the current end-of-input behavior.

Written by the indexing model from the issue text.