Reset mail count when Thunderbird Window has been shown

[tbertels]

Is your feature request related to a problem? Please describe.
When not using the API count method (which slows down Thunderbird on my system), we have to open a different folder then go back to inbox to update the new mail count.

Describe the solution you'd like
Reset the mail count once Thunderbird has been shown aka restored (which means we've checked the new mail).

Describe alternatives you've considered

Additional context