New UX of welcome view #540

Todo Items:

• Implement a welcome page + tour page. In welcome page, provide shortcuts in group. In tour page, take a tour of main features. (PR initialize welcome webview #535)
• only pop up tour page when open in first time. (PR only pop up tour page for the first time #544)
• switch to welcome page automatically after finishing the tour. (PR navigate to welcome page after finishing feature tour #541 )
• assign proper screenshots in the tour page. (PR add screenshots for tour steps #546)