Vertical scrolling for text-editor.

Vertical scrolling for text-editor.

* Text can be vertically scrolled by panning on the text,
  panning the handles, keyboard's cursor keys, and adding
  or removing text.

* Smooth handle panning feature implemented. Allows the handle
  to follow the finger instead jumping to the next cursor
  position.

* Cursor position issues fixed when there are lines with
  different heights.

* TODO: Fix the highlight box visibility and the text's option popup.

Change-Id: If00fbfd4f9e0cd065a6d2b6c908b9a56f0b3a480
Signed-off-by: Victor Cebollada <v.cebollada@samsung.com>