text: Fix set_surrounding_text request

text: Fix set_surrounding_text request

Add cursor and anchor positions as arguments to the set_surrounding_text
request. The cursor and anchor positions are relative to the surrounded
text, so it does not make sense to have that separate. Remove the
separate set_cursor_index and set_selected_text requests. Also update
the corresponding event in input-method-context and add support for it
in the weston example keyboard.

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