text-input: Use cursor begin and end in set_surrounding_text
In set_surrounding_text cursor and anchor is used currently. Change it to cursor_begin and cursor_end instead, for the following reasons:
- Avoids misinterpretation of anchor, as some clients may treat selection start as anchor while others may treat it as selection end.
- Makes the terminology consistent with preedit_string, which uses cursor_begin and cursor_end.