Adapt latest UIKit API (as of Xcode 10.1)

[rikner]

Type of change: Maintenance and API compatibility

Motivation (current vs expected behavior):

There were some small API Changes in Apples UIKit introduced with Mojave / Xcode 10 / Swift 4.2. To me it seems like Apple adapted our CALayerContentsGravity ;-)

Please check if the PR fulfills these requirements

• [x] Self-review: I am confident this is the simplest and clearest way to achieve the expected behaviour
• [x] There are no dependencies on other PRs or I have linked dependencies through Zenhub
• [x] The commit messages are clean and understandable
• [x] Tests for the changes have been added (for bug fixes / features)

[michaelknoch]

merge? @rikner