fix(dashboard): Unify widget icon colors and document it's behaviour

[nickvergessen]

Summary

Currently apps provide different icons and backgrounds via CSS classes. Some dark, some white. Some automatically invert the color based on dark mode, some don't, …
This caused endless fixing of icons and the general framework multiple times, so now here is the final outcome:

• getIconUrl:
  Get the absolute url for the widget icon (should be colored black or not have a color).
  The icon will be inverted automatically in mobile clients and when using dark mode
• getIconClass:
  The icon will be inverted automatically in mobile clients and when using dark mode.
  Therefore, it is NOT recommended to use a css class that sets the background with:
  var(--icon-…) as those will adapt to dark/bright mode in the web and still be inverted
  resulting in a dark icon on dark background.

Also the widget ID is used as css class, so I documented limitation of characters and log a debug if it's not matching

• getId:
  Get a unique identifier for the widget
  To ensure uniqueness, it is recommended to user the app id or start with the app id followed by a dash.
  @return string Unique id that identifies the widget, e.g. the app id. Only use alphanumeric characters, dash and underscore

Screenshots

Before	After

Apps that need fixing

• Documentation at fix(developers): Clarify dashboard widget icons documentation#11996
• Photos: fix(dashboard): Fix ID and provide icon url photos#2542
• Mail: fix(dashboard): Fix dashboard icon mail#9843
• Recommended files: fix(dashboard): Fix dashboard icon recommendations#767

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[jancborchardt]

Very nice design fix, thank you! (Screenshots checked, not tested)

[AndyScherzinger]

/compile amend/

[nickvergessen]

/backport to stable29