Use relative paths to handle component icons

[piotr-cz]

This makes components use base name as an icon, instead of full URL.
Required for agentejo/LayoutComponents#1

[aheinze]

I'm not sure about that. this PR would limit the ability to use custom icons 🤔

[piotr-cz]

Actually it unlocks the ability to use custom icons by ability to define icon base name (ie.business.svg)
instead of absolut url which is required at the moment (/path/to/cockpit/assets/app/media/icon/business.svg).

However developer won't be able to define custom icons which are outside /path/to/cockpit/assets/app/media/icon/ directory.
Compromise would be ability to define icon file as relative url to cockpit base url (./assets/app/media/icon/business.svg)

[aheinze]

I think a compromise would be to introduce a pattern like icons:code.svg for icons located in /assets/app/media/icons/ otherwise handle it as a link.

[piotr-cz]

You mean by adding a system path, that would be used as icons directory?

$app->path('#icons', COCKPIT_DIR . '/assets/app/media/icons');

[piotr-cz]

I think adding new path for icons (so it's possile to use icons from outside of /assets/app/media/icons) other feature out of scope of this PR, there would have to be changes in any module that uses icons (collection, forms, singletons).

In latest commit I've changed paths handling so it's working with icons in similar way how collection module is:

App.base('/assets/media/icons' + icon) -> '@url('assets:app/media/icons/')' + icon.

---

Update: Ah, I forgot that the @url expression is not available in frontend, so reverting latest commit

@aheinze How can one use custom icons now? I didn't know it's possible.
Actually I created this PR use custom icons for layout component