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

[Ximi1970]

I found a bug preventing the app from clearing the new mail icon. It now follows the new mail indication like TB. As long as there is a new mail marker in the TB folder, the new mail icon stays "active".

[Ximi1970]

New release.