Support for hot-swapping batteries and allowing missing batteries without errors

[Nukesor]

Hi!

First of all, thanks for providing this project! Looks great and it really has a super low memory/processing power footprint.

This PR is a follow-up to the other two existing battery PRs.
#233
#585

The idea of this PR is to allow several new things, but all visible changes will be hidden behind configurable feature flags.

The PR is based on #585. Thanks to @flying7eleven for providing an easy entry point to this whole issue.

There are three issues, each addressed in it's own commit:

1. Users should decide, whether they want to ignore if a battery is missing or not. The current error message kills the whole bar and forces the user to reload i3.
   This issue has been solved by introducing a new configuration flag allow_missing_battery. If set to true, there will be a simple placeholder text instead of crashing the whole bar.
2. Users should be able to completely hide the widget, if a battery goes missing.
   This is something that comes from people using the same configuration accross several devices.
   I myself, for instance, have two laptops (one at work, one at home) and one of them has a single battery, the other one has two.
   This change simply enhances the ergonomics for users with multiple devices.
3. Hot-swapping batteries.
   This needed a few more changes, since some values of the battery were previously only read once during initialization.
   Since hot-swapping can change the specs of a battery device, these values now have to be read continuously.

GIF of a hot-swap in action.

GIF of a hot-swap with hide_missing = true

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[Brod8362]

This is great! I was just thinking of this the other day, having a battery that may not always be present shouldn't break the entire bar.

[ammgws]

Yeah I think this is the best we can do, thank you @Nukesor and @flying7eleven for the clear overview and work on this issue. Perhaps it would be nice to add another format string to use when allow_missing_battery is turned on so one could have it display something other than "X%" or "xx:xx" when the battery is missing (like the KDEConnect block's format_disconnected option).

[ammgws]

Also could you share a gif or image of what the bar looks like with hide_missing is used?

[Nukesor]

Hey :)
I renamed the parameters from *_missing_battery to *_missing. Those are indeed better names!

On top of that the new missing_format string has been added. This has been tested and works as intended.

I also added a few missing parameters to blocks.md in this commit.

A gif of a hot-swap with hide_missing has been added to the original post :)

[ammgws]

LGTM

Thanks!