Add simple filter rule to add tags to incoming messages via Sieve

[djast]

Is your feature request related to a problem? Please describe.
It would be useful if custom tags could be applied to messages via a simple filter action in Settings -> Filters if the IMAP server supports the imap4flags extension.

Describe the solution you'd like
Under Filters -> Settings -> rainloop.user -> Add a Filter, present "Tag with" (or "Add tag(s)", "Apply tag(s)" or whatever wording you prefer) as a possible Action; when selected, display the list of checkboxes for custom tags to be applied (as it appears in the tags dropdown in the message pane).

Describe alternatives you've considered
This appears to be possible using an Advanced filter, by adding

require ["imap4flags"];

to the script, and defining a rule with an action such as

addflag "Work";

but it seems useful enough to make available via the Simple filter mechanism.

[the-djmaze]

#80 (comment)

The current simple editor will stay as-is to be compatible with RainLoop users.