Stop using UIScreen.mainScreen to fix accent color

[pixlwave]

What does this PR do? UIScreen.mainScreen is deprecated and SwiftUI apps lose their accent colour if the screen is accessed this way before the app's body is evaluated. This PR switches to reading the screen size from the app's first window and fixes #53.

Where should the reviewer start? Test it against a sample app.

How should this be manually tested? Read the steps to reproduce the issue in #53 and test this PR against that.

Any background context you want to provide? No

What are the relevant tickets?

53

Screenshots or screencasts (if UI/UX change)

Questions:

• Does the docs need an update? No
• Are there any security concerns? No
• Do we need to update engineering / success? No

[benjackwhite]

Going to check this out locally as I don't think this generally works. I believe that sharedApplication is not available in extensions which we technically currently support.

[pixlwave]

Ah yeah good point, didn't think about that 🤔

[pixlwave]

Closing in favour of #55.