New UX of welcome view

[Eskibear]

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)

[Eskibear]

Telemetry

• telemetry for actions (external links, button, tab-switching...) finalize telemetry in welcome view #553

When there's no open folder:

• Proper guide in Java Project Explorer, instead of an error message. @jdneo
• Proper guide/infor in Testing view. @jdneo

visibility of welcome view (@Eskibear )

• Hide in Codespaces by default.
• Controlled by A/B testing