Add dialog-box warning support

[spacegaier]

Breaking change

Proposed change

Helper function to create a question dialog with default Yes/No option (no button selection so user has to purposely select an option). The confirm and dismiss texts can still be override as before.
Also added the option to flag a dialog as containing a warning so that the matching CSS class gets applied.

The idea came to me when I changed the name of a device. The user then gets asked whether the entity IDs should also be changed. I saw two issues here:

1. It was a Yes / Cancel dialog, but you cannot actually cancel anything here (the device renaming already happened, so Yes/No is correct IMHO).
2. Clicking Yes here has a comparably big impact and to make sure a user is aware of the potential ramifications, this should be visually highlighted.

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:
• Link to documentation pull request:

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