add android example - calculator

[1595901624]

This is a Android calculator example in Slint 1.3.0

[CLAassistant]

All committers have signed the CLA.

[FloVanGH]

Hi, thank you very much for the PR. @ogoffart is the best to review this PR, because he is working on the Android port. He will be back next week.

[ogoffart]

Thank you for your contribution, and my apologies for the delayed response.

The calculator example is a great addition, but before we merge it, there are a few issues to address:

• The CI is flagging a license header issue. We can either update the files with the expected copyright header or modify the CI script to allow different names in the copyright line, which is something we've been considering

• Hardcoded Button Height: I noticed the button height is hardcoded. Consider using a percentage or rem value.