Allow users to show the Tags in the folder pane by exposing a simple toggle in the meatball menu of the Folder Pane Header.

This option should also be exposed in the Menu Bar inside View > Folders.

All these options should be covered by tests.

Depends on D178910

… where "all" means all folders in all accounts except trash, junk, outbox and virtual folders.

For these special virtual folders, choosing the folders to search will happen every time the
virtual folder is opened, instead of when it is created, avoiding problems keeping up with changes
such as new account creation.

Depends on D179327

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/631d07168f8d
Implement a Tags folder pane mode. r=aleca
https://hg.mozilla.org/comm-central/rev/b6272f0184a4
Test Tags folder pane mode. r=aleca
https://hg.mozilla.org/comm-central/rev/a1fdac85db2d
Allow virtual folders to search all folders. r=aleca

When testing the fix for bug 1831759 comment 14 I noticed that after I tagged a couple of random messages as "Important" in an imap folder, I had to toggle the "Quick Filter" button for them to appear in the "Important" smart tag folder. The number of messages listed for that folder was correct (2) but to see the two messages in the "smart tags folder", I had to click the "quick filter" button. There was nothing set in the search fields of quick filter that would affect this, it seemed.
Also, I set some "Later" tags in Local Folders and they never appeared at all in "Later" smart tag folder.
Anyhow, maybe I'm doing something wrong or there is more work needed for this feature?