Tag by label

And another one :-)

Tag a thread by label in any ThreadIndexMode. My use case: Tons of dev emails from different clients (github, gitlab, appsignal, newrelic, sentry), sometimes I want to tag a subset and do whatever, without switching the buffer.

Couple points:

* Again the issue of tests; the method is trivial but I prefer to test my code (besides a user test). Preferred? Pointers?
* Keybinding O is ridiculous. I wanted to go for ctrl_t but couldn't get it to work (no binding in ncursesw and couldn't get the ord)
* I figured tagging a single label would be enough, otherwise you might as well do a more complex xapian search (given my usecase), but yours might differ, if more complexity is needed I'll fix it up.