Remove isDestructive button input

[RasmusKjeldgaard]

Which issue does this PR close?

This PR closes #2737

What is the new behavior?

Destructive buttons are now only enabled as a class that can be applied on the attention-level 1 button and is not explicitly exposed to consumers via an input. This is because the destructive button should only be used inside an alert, as per the UX spec.

Alert applies the class now instead of setting the input.

This paves the way for a simpler theme concept, as isDestructive will not have to be handled for each attention level.

Does this PR introduce a breaking change?

• Yes
• No

The isDesctructive behavior of button should only be used internally by Kirby in the alert, as destructive actions are always announced via an alert. Use the default button attention levels and combine with alerts when needed.

Are there any additional context?

Checklist:

The following tasks should be carried out in sequence in order to follow the process of contributing correctly.

Reminders

• Make sure you have implemented tests following the guidelines in: "The good: Test".
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Review

• Determine if your changes are a fix, feature or breaking-change, and add the matching label to your PR. If it is tooling, dependency updates or similar, add ignore-for-release.
• Do a self-review.
• Request that the changes are code-reviewed
• Request that the changes are UX reviewed (only necessary if your PR introduces visual changes)

When the pull request has been approved it will be merged to develop by Team Kirby.

[mark-drastrup]

Nicely done!