[Tizen] Refactor XWalkLauncher for usage of RESET event.

After this change execute xwalk-launcher on Tizen will cause
initialization of app core and launch of g_main_loop. Application
will be actually launched after xwalk-launcher receives RESET
event from platform.

BUG=XWALK-2779
wrong initialization order. Cannot you get main_loop from the base class?