Add option to show an alert before dismissing modal

[mark-drastrup]

Which issue does this PR close?

This PR closes #376

What is the new behavior?

It's now possible to pass an alert config as an optional 3rd argument to the modalController.showModal method. This will trigger an alert, when the user tries to close a modal and make them confirm whether or not it should be closed.

Does this PR introduce a breaking change?

• Yes
• No

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.

[mictro]