[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
the file location rules are following: if the file makes platform specific customization of a common functionality than it's located in the same folder as generic file but has "_platform" postfix. If the implements something platform-specific stuff which does not have generic implementation, it should be put to "platform" folder.

In this case we have Linux-specific shared process model implementation, so we have xwalk_launcher in "linux" folder, on Tizen however some more tricks are needed, so we have xwalk_launcher_tizen for it.