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
I think it's a good practice to implicitly keep the empty ctor & dtor instead of removing them. And I also noticed other classes in the source code have the empty ctor & dtor as well, such as the PermissionsInfo, CSPInfo, etc.
So I suggest to keep them but move them into the cc file. What do you think?