feat: setting to show battery percentage in the panel

[SammyMcFly]

The changes add a setting to have the cosmic-applet-battery show battery percentage in the panel.

This fixes one of two request from #617.

The title of the added setting is up for discussion. I added an English draft-title: "Show percentage", however, this might be too short or not descriptive enough. Perhaps "Show battery percentage in panel" might be better. Let me know what you think!

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[leviport]

Since this is a UI change, it will need approval from the UX team. I know the longer term strategy was to have this setting in applet settings once the applet settings system gets built out, but this seems like a good stop-gap in the meantime. It exposes the option in GUI, and there is definitely a lot of interest in the setting.

Here is a screenshot of the Power & Battery page with the new setting:

From a quick test, this is working as expected. If UX is okay with the change, I'll run through regression testing.

[SammyMcFly]

Since this is a UI change, it will need approval from the UX team. I know the longer term strategy was to have this setting in applet settings once the applet settings system gets built out, but this seems like a good stop-gap in the meantime. It exposes the option in GUI, and there is definitely a lot of interest in the setting.

Good to know. Will this be an extra page in cosmic settings or just a setting inside the applet? The latter I could take care of, for the former you will probably have a design laid out that I am not aware of.

[leviport]

I believe the current plan is a page in Settings.