Refactor Panel package

[distantnative]

Description

Certainly a PR with a lower priority

Summary of changes

• Moved some classes into sub-namespaces, e.g. the dialog classes to Panel\Ui\Dialogs\
• Refactored many classes to be less static, especially the Panel\Panel and Panel\View classes
• Moved functionality in dedicated classes
  • Panel\Areas for the logic around gathering and resolving area definitions
  • Panel\Fiber handles all of the Fiber data wrangling incl. globals
  • Panel\Access took over the firewall part (mainly just to have that in one place that's not making Panel\Panel larger)
  • Panel\Router takes care of the routing and general response for the Panel
• Removed the $ key prefixes
• New $kirby->panel() that returns the Panel\Panel object

Reasoning

The labyrinth of static method calls was just so hard to parse and some classes (Panel and View) were getting way to big

I also wanted to get a way to access much of this via the $kirby class. So I needed to start a non-static chain of objects.

Additional context

Are there ways to make this easier to review?

Changelog

Fixes

Breaking changes

• Fiber data: keys aren't prefixed anymore with $. This also affects Panel plugins reloading the Panel by defining only specific keys to be reloaded.
• Many Panel package classes are now non-static. If you are using them in your code directly, you'll need to update your call signatures.

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add changes & docs to release notes draft in Notion