Format Bar: Animate item changes

[frosty]

Refs wordpress-mobile/WordPress-iOS#7763 in WPiOS

This PR adds animation when switching the format bar's defaultItems and overflowItems collections.

To ensure a smooth transition, I've made the the defaultItems and overflowItems properties read-only and added a new setDefaultItems(_:overflowItems:) method to set them both at the same time.

To test:

• Check out the associated WPiOS integration branch: https://github.com/wordpress-mobile/WordPress-iOS/tree/feature/media-format-bar-animations
• Open Aztec
• Tap the + button in the format bar. The new media buttons should animate in.
• Tap the X to cancel out of media insertion mode. The text formatting buttons should animate in.
• Ensure the animations look correct both when the bar's overflow items are expanded and collapsed.

Needs review: @SergioEstevao

[SergioEstevao]

Code looks good, new animations are excellent!

[frosty]

Thanks @SergioEstevao!