[Tizen] This commit implements the splash screen for Tizen.

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
I understand, actually I cannot pass the valid host widget to SplashScreen in the constructor since the host widget is not created here. Instead the host widget is created in the Initialize(). So I have to have SetHostWidget() called in the Initialize().
I agree that the code looks weird. I think the proper place to create SplashScreen is in Initialize() instead of the constructor. I will refactor a bit more and let you review. Thank you @pozdnyakov