This implements the IPC binding event for keyboard bindings. It is
slightly different from the i3 implementation [1] since sway supports
more than one non-modifier key in a binding. Thus the json interface has
been changed from:
{
...
"symbol": "t",
...
}
to:
{
...
"symbols": [ "t" ],
...
}
[1] http://i3wm.org/docs/ipc.html#_binding_event
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