Final Version of welcomeWindow design & adding a lot splash screens and app icons

Enhancements including the following:
 - Replacing the welcoming text with the logo
 - Adding gradient background for android and iOS
 - New style for continue button
 - Adding all needed splash screens (adding a new folder named platform
in the root directory to handle android splash screens)
 - Adding all needed app icons
 - Removing needn’t folders and images (eg. tizen)