Manual Refresh

[marbetschar]

This PR add the ability to manually trigger a refresh and fetch messages from backend:

However, two Gtk issues remain with the current implementation - wasn't able to figure out what's going on here:

• The refresh button seems to be active and the default action whenever Mail is started. How to change this? setting can_focus = false fixed this
• The animation of the state change seems to be started quite late, not right on click. How to improve? yield ..async ... fixed this - thanks @dbhowell!

And one UX question: I'd love to use <Control>R for refresh/reload - but it's already used for Reply. Thoughts?

[danirabbit]

We can probably look at default widget focus in a follow up branch, but I think it makes sense to get this functionality in :)