Fix showing context menu on elements with style "-webkit-user-select:none".

Fix showing context menu on elements with style "-webkit-user-select:none".

[Issue] Elements which are not selectable, should also not show context menu
  on long press.
[Problem] Redundant showContextMenu() being called whenever LongPress ended.
[Solution] Trigger context menu only when neccessary.

[Requirment] WCS TC 49,50
[SPIN JIRA] TNEF-306
Reviewed by: Antonio Gomes, Jaesik Chang, Marcin Kolibabka, Piotr Grad

Change-Id: I841075559f2a82b94d5b4e3ecbcd485b5e119553
Signed-off-by: Bartlomiej Gajda <b.gajda@samsung.com>