This commit implements the splash screen for Tizen only.

It read a splash screen image (static) from the wgt config file and show it on the center of the main window when the application is launched.
And after the content is loaded, hide the splash screen image in amincation way.

The wgt config has a setting node to define the basic configuration of the splash screen. And currently it contains the path of the image as below:
&lt;splash-screen src="splash.png"/&gt;

BUG=XWALK-1150
would be mo readable if 
const std::string& splash_screen_path = create_params.splash_screen_path;
if (!splash_screen_path.empty())..