Correct ContextPopup based on review comments

Correct ContextPopup based on review comments

- Improve code example to show DirectionPriorities usage.
- Remove unnecessary code from code example.
- Enhance DirectionPriorities description.
- Remove icons which are named "menu/xxx" because they are the same as
  "xxx" currently.
- Allow ContextPopupIcon label to be null. This is so that
  ContextPopupItems which only use icons can be created.

Signed-off-by: Sidharth Gupta <sid92.gupta@samsung.com>
Change-Id: I8578f102015c9e095f051d5202cff8eb8e0044c7