Support for non-font-awesome icons for tabs

[UncleSamSwiss]

Is your feature request related to a problem? Please describe.
The "problem" (if we want to call it this way) is that many adapters have started using icons different from font-awesome for their tab icon. One example is zigbee:
https://github.com/ioBroker/ioBroker.zigbee/blob/dbe52d251fca2e767ff860d4b449286ab23835c2/io-package.json#L183

See also the discussion here: ioBroker/ioBroker.zigbee#1068

Describe the solution you'd like
I would prefer if admin 5 supported something like this:

        "adminTab": {
            ...
            "icon": "zigbee.svg"
        }

This icon would have to be uploaded to the admin folder of course.

Suggestion: it would be nice if there was a requirement for it to be a "themeable" SVG icon (using the special color currentColor), so it is more in line with the FA icons of other tabs (ie monochrome). Here are examples how this can be achieved: https://github.com/UncleSamSwiss/iobroker-dev-portal/blob/1cc4d5b10083b37c95c04e326886e096a2c7aa15/express/frontend/src/components/Icons.tsx#L27
(it doesn't have to be a JSX/TSX file, the same works for pure SVG files).

Describe alternatives you've considered
The current (ugly) alternative looks like this for zigbee:
ioBroker/ioBroker.zigbee#1068

Additional context
Have a look at the many adapters that are using their own icon:
ioBroker/ioBroker.zigbee#1068 (comment)

[GermanBluefox]

font-awesome are not supported any more in Admin5. You can use base64 svg icons with currentColor.
Here is an example: https://github.com/ioBroker/ioBroker.js-controller/blob/master/io-package.json#L324

And it will be shown as
<svg xmlns="http://www.w3.org/2000/svg" height="24px" viewBox="0 0 24 24" width="24px"> <path fill="currentColor" d="M12 6c1.1 0 2 .9 2 2s-.9 2-2 2-2-.9-2-2 .9-2 2-2m0 10c2.7 0 5.8 1.29 6 2H6c.23-.72 3.31-2 6-2m0-12C9.79 4 8 5.79 8 8s1.79 4 4 4 4-1.79 4-4-1.79-4-4-4zm0 10c-2.67 0-8 1.34-8 4v2h16v-2c0-2.66-5.33-4-8-4z"/> </svg>

[UncleSamSwiss]

font-awesome are not supported any more in Admin5. You can use base64 svg icons with currentColor.
Here is an example: https://github.com/ioBroker/ioBroker.js-controller/blob/master/io-package.json#L324

@GermanBluefox The example you provided is for an object. Is the same true for the admin tabs?

[GermanBluefox]

Admin tabs may not have own icons. The adapter icon will be used for that: