Fixes initial frame issue

[dtrotzjr]

The first time the Message Banner is shown it slides in from an arbitrary position at the top left. As far as I can tell this is due to the constraints being applied outside of the animation block.

Also setting currentMessageBanner.center = target is really not needed as this is using constraints which override setting the center anyway. I would drop setting currentMessageBanner.center = target as well as calculateTargetCenter: altogether except that there are side effects in calculateTargetCenter: which are needed.

[thibault-carpentier]

Thanks for the PR, il will merge a rework the center value during this weekend.

[dctrotz]

Awesome! Thanks!