Ensure the cursor rectangle is updated when preedit text changes.

Updating only when the cursor position changes isn't enough because
changing pre-edit text changes the width of the pre-edit area and
the x offset of all cursor positions within it.

Change-Id: I3c0a5e4ad4714a903ca84c1a25374491f34d95a0
Reviewed-by: Yann Bodson <yann.bodson@nokia.com>