Add specific condition for back-key handle

Add specific condition for back-key handle

[Issue#]     P130829-05787
[Problem]    Long press on the text to enter text-selection mode.
             Back-key is working to history back or send tizenhwkey event to Application even current mode is text-selection mode.
[Cause]      Elementary is called back-key callback both webkit and wrt.
[Solution]   WRT calls "ewk_view_text_selection_clear" before handle back-key.
             If return value is "EINA_TRUE", back-key is handled by webkit.
             As back-key event is handled by webkit, WRT doens't need to handle back-key.
             In case of return value is "EINA_FALSE", WRT should handle back-key.
             History back or dispatch tizenhwkey event to application.
[SCMRequest] N/A

Change-Id: I3ecea8ae5c3aaec71fdd07b86b6df49e70dae500