[Improve base View][1/n] Add subview handling to View

[taneliang]

Stack PR by STACK ATTACK:

• #99 [Improve base View][1/n] Add subview handling to View

• 100 [Improve base View][2/n] Rationalize view layout system

• 101 [Improve base View][3/n] Add _ to views' private vars and methods

• 102 [Improve base View][4/n] Write tests for geometry.js

Stacked on #96.

Summary

This PR begins a stack of PRs that improves the base View class implemented in #80.

This PR adds subview management to View, as there was a lot of duplicated subview handling code present in almost all our View subclasses. This also brings us closer to UIKit's UIView, which also handles its own subviews.

Resolves #95.

Test Plan

• yarn start: nothing broken
• yarn lint
• yarn flow: no errors in affected code
• yarn test

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