display: make Key Menu reflect real bindings

[CosmicToast]

This changes the keybinding menu (alt-g) to reflect real keybindings.
Potentially contentious things:

• internal/display/util.go instead of maybe somewhere in config
• exact details of the "friendly" function
• exact API

This also patches the $(bind:) special of the status line.
Why does it need patching?

In go, when you iterate a map, the order is non-deterministic.
With the current handling of $(bind:), if you have >1 binding for the action, which one you get is essentially random.
The way you solve this is to ensure a deterministic order by sorting the set of keys.

This needs to happen every invocation, in case the user changes a binding.
An alternative method would be to keep a cache that is updated whenever bindings are written to.

Lack of this specific feature is a commonly cited reason (to me, personally) of micro not being adopted (thus why it doesn't show up in the issues - it's non-users as of right now), so I figured I would at least offer a way to fix it.
There's already a TODO for it (I didn't remove it in this patch, see next paragraph), so it seems like something that's desirable.

I don't really have the time or energy to modify this patch to suit the exact needs/requirements for merging.
However, it can/should be reasonably trivial to move it anywhere / change some of these minor specifics.
Another thing that could be brushed up (perhaps later?) is the exact way keydisplayGen performs the generation.
Either way - please consider this code to be CC0/Unlicense/BSD0/Blue-Oak-Model 1.0.
I'd rather see it (as in the feature/behaviour on the overall) get merged, ideally with as little involvement from my end from this point on :)