Add get_bindings IPC interface

The new get_bindings IPC interface (IPC_GET_BINDINGS) allows swaymsg (and
other IPC clients) to interrogate sway's input bindings for all configured
binding modes.

The output of get_bindings is a JSON object mapping each configured binding
mode (e.g. "default", "resize", etc.) to an array of binding objects. The
binding JSON objects are of the same form as those in IPC_EVENT_BINDING.

N.B. a big chunk of code from ipc_event_binding() is moved to a new
ipc_json_describe_binding() function which services both the new
IPC_GET_BINDINGS as well as the existing IPC_EVENT_BINDING commands.

Signed-off-by: Peter Grayson <pete@jpgrayson.net>

completions/bash/swaymsg

@@ -15,6 +15,7 @@ _swaymsg()
     'get_bar_config'
     'get_version'
     'get_binding_modes'
+    'get_bindings'
     'get_config'
     'send_tick'
   )