Fix Shift + Left/Right key behavior

- When the cursor was at the end of the text, the text selection couldn't release again by keyboard.
  Fixed to make deselection enabled wherever the cursor is.
- If Left selection handle is at the same position as Right selection handle,
  need to remove Highlights box between them.
- When all text is being selected using Shift + Arrow key and then only Arrow key is pressed,
  1. the text selection is released,
  2. the event state changes to 'EDITING' mode, and
  3. mPrimaryCursorPosition, mLeftSelectionPosition, and mRightSelectionPosition
     should be at the same position.
- When the text is selected by using Shift + Arrow keys or by tapping, if press Left or Right
  arrow keys, the cursor should be at the beginning or the end of the selected text (depending
  on whether it is the left or the right arrow key that is pressed).

Change-Id: I0d7869c0231c52296e37a3268dffe3e707de8cbd
Signed-off-by: Seoyeon Kim <seoyeon2.kim@samsung.com>