First step of email actions

Just to have some feedback before going further
Please keep in mind that some people are not using GMail ;)
GMail has no "folders", only labels. But in every other mail services, copying and moving messages across folders is not the same as tagging them.

I tried to refactor the `mailbox-list` component, but this wasn't very satisfying. `mailbox-list` is like a select box displaying the selected folder. Here I want a menu, it's not really the same UI component.