feat(fab-button): add close button icon property

[anagstef]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

When a ion-fab is used with a list inside it. If you activate it, then a close icon appears inside the ion-fab-button that cannot be changed.

What is the new behavior?

This PR exposes a new property to be able to change the icon shown when fab is activated. Defaults to the previous one: close.

Does this introduce a breaking change?

• Yes
• No

Other information

[brandyscarney]

Thank you for the PR, this is great!