Gradle build improvement

[carlosame]

This is an initial fix for the Gradle build (I ran it with Gradle 7.5.1). The build is not perfect but is a better base to work on, depending on the maintainer's technical decisions. (For example I'd personally get rid of the Proguard task)

If one runs gradle build -x test it completes just fine.

Remaining problems:

1. The getPhysicalFontReturnsPhysicalFontForFontFamily test fails.
2. The (now commented out) license plugin fails due to a number of missing headers. Once the problems are fixed, could be uncommented.
3. The creation of Jar files still need a few adjustments (which I didn't do because of the interaction with the Proguard task).
4. gradle cobertura coveralls fails with a number of errors, and the coveralls-gradle-plugin is not maintained anymore.