Changing maven compiler plugin config. Updating UICore singleton.

[mriss]

Specs for <compilerArgs> found at: https://maven.apache.org/plugins/maven-compiler-plugin/compile-mojo.html

[mriss]

UICore is instantiating singleton inline at the class init code. That make dificult to run tests/integration tests using headless approach in CI/CD environments, sinde UICore uses UI initialization inside the constructor. The inline initialization is making difficult to mock UICore class. Using GuiUtils calls is not enought, since it returns UICore. Thus, we need to mock GuiUtils static method to return a UICore mock, avoiding to enter inside UICore details during headless tests.

[nroduit]

This singleton pattern is not thread-safe (really required by Weasis). See https://www.geeksforgeeks.org/java-singleton-design-pattern-practices-examples/

[nroduit]

I published a fix to be thread-safe. Let me know if it helps for mocking.