Show snackbar when tab was opened in the background

[pocmo]

For tabs opened from the context menu:

• Normal tab: As a user I want to be able to long press on any link in web content to open it in a new tab #85
• Private tab: As a User I want to long press on any link in web content to open it in a new private tab #86

I'm not sure if we will be able to handle this in a component since the snackbar needs access to a view in the hierarchy to attach itself to. So for now I filed this here - but maybe we actually will handle this in the context menu feature (mozilla-mobile/android-components#1386)

[jonalmeida]

Closing this as fixed since it was implemented in the context menu component.