text: Split out cursor_position

text: Split out cursor_position

Add an extra cursor_position, which also allows to change the anchor
(for slections). Change the index type to int to allow setting it before
the beginning of a commited string.

The cursor should not be moved as a direct repsonse to this event but
atomically on the next commit_string event.

Signed-off-by: Jan Arne Petersen <jpetersen@openismus.com>