Improve the test application

[mahozad]

Add every property of the chart to the test app screen as buttons, sliders, etc. so that the chart can be modified to get immediate feedback and to design a desired shape, color, ... for the chart and also to test all combinations of properties of the chart manually if required.

Also, create a java class and test the library in it to see if it works with java as well.

• [x] Rename the test subproject to something more appropriate like showcase or preview or demo or exhibition.
• [ ] Use ComponentActivity as the base class of the activities. See https://developer.android.com/reference/androidx/activity/ComponentActivity
• [ ] Mention in the README that "Each release is shipped with the showcase APK. You can download it in the assets section of each release."