Fix for pre-edit text.

Fix for pre-edit text.

* When editing with predictive text enabled, the imf manager removes
  the current pre-edited text and inserts the new string.
  The new string can be longer if text is being added,
  or smaller if text is being removed.

  The text needs to be scrolled to the cursor position if text is smaller.

* Similarly, the text-controller can remove a group of characters when a
  ImfManager::DELETESURROUNDING event arrives. The text also needs to
  be scrolled to the cursor position.

* If text is not added at the end of the string, the text must not be scrolled
  unless the cursor becomes not visible.

Change-Id: I6ed4b9ea122cd0b8fac4111c732625bf636eb52d
Signed-off-by: Victor Cebollada <v.cebollada@samsung.com>