Slides: change showNavigation default to true

[RasmusKjeldgaard]

Which issue does this PR close?

This PR closes #3209

What is the new behavior?

Navigation arrows and slide indicator is now shown by default on kirby-slides.

Does this PR introduce a breaking change?

• Yes
• No

Migration notes added to MIGRATION.md.

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

• 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.