Resize UIWindow subviews' frames to window bounds

[ephemer]

Type of change: Graphical improvement

Motivation (current vs expected behavior)

We want to make UIWindow resizable depending on external factors. For example - by resizing the window on Mac, or by showing/hiding the status bar on Android, entering/leaving immersive mode etc.

Please check if the PR fulfills these requirements

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

[codecov[bot]]

Codecov Report

Merging #311 into master will decrease coverage by 0.07%. The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #311      +/-   ##
==========================================
- Coverage   51.42%   51.35%   -0.08%     
==========================================
  Files          87       87              
  Lines        2872     2878       +6     
==========================================
+ Hits         1477     1478       +1     
- Misses       1395     1400       +5     

Impacted Files	Coverage Δ
Sources/UIWindow.swift	61.11% <0.00%> (-7.64%)	:arrow_down:
Sources/UIScreen.swift	11.42% <50.00%> (+0.74%)	:arrow_up:

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[ephemer]

I just had a look through the updated code and other than the code comment that I suggested we add this looks great to me 🚀 Feel free to press the button when you're ready!