## Description
This adds `find.backButton()` in the Common Finders to allow finding different types of standard UI elements. It works by attaching a key made from an enum value in a new enum called `StandardComponentType` to all of the standard widgets that perform the associated function.
I also substituted the finder in several places where it is useful in tests.
This allows writing tests that want to find the "back" button without having to know exactly which icon the back button uses under what circumstances. To do it correctly is actually quite complicated, since there are several adaptations that occur (based on platform, and whether it is web or not).
## Tests
- Added tests.
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