Add basic Gradle build file #49

New Gradle and old Ant build files exists in parallel. Gradle build
script cannot yet build all artefacts though. Therefore, `build.xml`
should be still used by default.
Use gradle to clean resources
Use gradle to build java classes
Use gradle to build java test classes
Disable "Mutation Coverage" target
Replace old build tasks with one gradle call
Replace old build task with one gradle call
Remove unused definitions from build.xml
Remove sonar.xml
Detect current version in gradle script
Replace "prepare-release" ant task with gradle one
Unzip swt.jar for windows32