Implement IPC command that returns current mode

[s-ol]

This would allow to add a mode-indicator to 3rd-party bars (for example a powerline segment would just use i3-py and call to something like get_mode).

[Airblader]

Can you please report this against i3? I don't see why it shouldn't be added there. However, I think it makes more sense to implement and event rather than a command as with a command constant polling would be necessary.

[s-ol]

okay, moved to i3/i3#1707