Add icon support to gauge

[spacegaier]

Breaking change

Previously there was no icon shown (inconsistent compared to other cards). Now, the icons will appear unless we add a toggle for that.

Proposed change

Icon now shown in front of label.

Editor support:

Also for a more consistent look with the entity and sensor cards (plus feels lighter / more modern), the label now uses the secondary color.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion:
• Link to documentation pull request: Add icon to Gauge card home-assistant.io#16323

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io