Add support for full offline mode

[Kabouik]

nmail can already queue messages or save them to drafts (later to be synchronized with server drafts) when the user tries to send one while offline, however toggling messages between read and unread doesn't work while offline. It would be useful to support that, and synchronize with the server whenever the connection is established.

Somewhat related: it's not clear to me what is the reconnection strategy when offline. It seems nmail tries to connect for some time, then after some rather long time-out period, it shows "offline" and stays like this. Also, when "connecting" and the Internet connection is restored, it takes a long time to nmail to actually reconnect. It is usually much faster to quit nmail then launch it again, so maybe the reconnection algorithm could be improved? Maybe a keybind to force a reconnection attempt and one to interrupt it would be good too, as reconnecting endlessly seems to eat more CPU, which can be important on mobile devices where network changes are a thing.

As part of full offline support, it would be useful to support offline-specific theming, so that it can be made clear when nmail has no connection, i.e., with a red top bar.

[d99kris]

I think there are many interesting ideas suggested, I can look into some of them. "Full offline" support is unlikely to get implemented though (I'll try to expand on the rationale for this at some point). But certainly I can look into improvements to reconnection handling and adding manual reconnect. As an interim workaround, if you trigger a refresh of current folder L, I believe it should trigger an immediate reconnect attempt.