Support Virtual Folders

[sesch29]

Is your feature request related to a problem? Please describe.
As a user, I want to also get statistics about Thunderbirds virtual folders.

Describe the solution you'd like
So I can write complex search requests, store them as a virtual folder and configure ThirtStats to watch that virtual folder. Currently I can already select the virtual folder, but no results are shown; so this feature could also be seen as a bug.

Describe alternatives you've considered
The Alternativ extension "ThunderStats" already supports virtual folders.

Additional context

[devmount]

Thanks for this feature request. The actual bug is, that virtual folders are shown in the select list, they shouldn't be selectable. We had a lot of issues caused by virtual folders (like incorrect email count, infinite processing etc.). When virtual folders are properly working with Thunderbirds WebExtension APIs we can give this a spin.