Fix missing fab-sheet backdrop on mobile devices

[mictro]

Which issue does this PR close?

This PR closes #2398

What is the new behavior?

Fab sets backdrop on mobile devices.

Does this PR introduce a breaking change?

• Yes
• No

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".
• Make sure you have updated the cookbook with examples and showcases (for bug fixes, enhancements & new components).

Review

• 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.

[RasmusKjeldgaard]

@mictro consider extending the 'What is the new behavior?' section with a description of what the problem with the existing code was, and why the setTimeout is no longer needed etc. Both for future us, and so I can better understand why this change does not break anything 😁

[RasmusKjeldgaard]

It seems like the opacity transition (fade-in effect) is no longer working for the backdrop with this change.

The setTimeout was probably there to make sure that the backdrop-visible class was added to task queue as a new task, and therefore would happen after the backdrop was already in the DOM, so the transition is applied.

[mictro]

Oops, timeout reintroduced and opacity animation is now visible.