Add shadow to keyboard keys

[andrewtavis]

Terms

• [X] I have searched open and closed design issues
• [X] I agree to follow Scribe-Android's Code of Conduct

Description

Once #24 is finished, the next step will be to add shadows to all keyboard keys. These changes will likely happen in MyKeyboardView.kt.

Contribution

This will become a priority after #24 is finished, and I'd be happy to work on it with people :)