[NcAppSettingsDialog] Allow to add icons to the navigation sections

[susnux]

☑️ Resolves

• Part of Improve settings #1441

This consists of 2 commits, the first one changes the way the sections are registered and the second on provides icon support.

Section registration

Previously the default slot content was iterated and the props of every vNode were used. This had two downsides:

1. $slots is not reactive -> changes will not propagate
2. We can not easily access slots of a section (like for the icon) because the slot will only be available when the vNode is mounted

So instead we now provide to functions to children:
registerSection and unregisterSection with that functions children (the sections) can register themself and update on change. Solving both issues: the reactivity and the ability to inject icons (as the registration is done after mount).

I personally also think it is easier to understand as no longer internals of vNodes need to be handled, but this is personal taste.

Icon support

There is a new slot icon on the settings section which allows to inject icons to the section name in the navigation.

🖼️ Screenshots

With this changes it loos like this:

without icons (looks like before)	with icons

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable