[GUI] Add a link to the "Looks like you are new here!" screen by a "?" icon in the navigation bar

[Necrosys]

Fixes #1016

Screenshots

• New wallet, Overview page, no transactions yet
• New wallet, Overview page, immature transaction only
• New wallet, Help page
• Old wallet, Overview page
• Old wallet, Help page

[seanPhill]

@Necrosys please supply a Veil stealth address so we can tip you for this PR (and the previous one).