Make actions shown in message view menu configurable

Fixes #5850.
Use named indices to get values from "visible refile actions" preference
Fix code style/formatting
Restructure code to set visibility of copy, move, archive, spam menu items

In my opinion this structure makes the code much easier to read.
Add icon for the copy action
Remove icons from the "Refile" submenu

We don't show icons in any other submenu.
Merge branch 'pr/364'

Make actions shown in message view menu configurable
Add visual indicator that a menu item opens a submenu

This isn't the best solution. Most of the developers agree that a right-aligned
triangle or arrow would be a better fit for such an indicator. But we currently
don't have the time to write the code to do it.
Personally, I think we should try to get rid of submenus altogether.
Run script to sync translations