Support keybind-based (instead of `ControlClient`-based) actions

[whisperity]

There are several valid use cases in which the user's shell (inside a session) is not able to communicate directly with the server that is executing said session. The two most important of such are when the user sus away (and no longer has write access to the socket) and when the session is running an SSH (or any other remote-ish) connection where the prompt the user can type in no longer has access to the socket. (In the latter case, even access to the monomux binary might be unreliable.)

We should support some rudimentary keybind-based (e.g., screen's or tmux's C-a) system where the user can bind a shortcut to a commonly used action.

• Action: Detach from the running session #3
• Action: Switch sessions "in-line" #4

[christianparpart]

The leader key (e.g. C-a should be configurable, and repeating it should act like passing that one through to the connected application. (just like in tmux). It should be possible to also bind the leader key to arbitrary keys, e.g. \ or <C-m> / <C-space>.