widget: Move IM cursor to the right position in im_update_cursor()
widget: Move IM cursor to the right position in im_update_cursor()

IM cursor is moved with pre-edit text.

https://bugzilla.gnome.org/show_bug.cgi?id=756141

saitoha/vte-sixel,saitoha/vte-sixel,saitoha/vte-sixel,thestinger/vte-ng,thestinger/vte-ng,thestinger/vte-ng