Reword source handling, remove configuration properties' defaults

• Rework handling of sources (to be included in the gwt-lib and in GWT classpath): delete the import-sources/import-test-sources goals and instead explicitly add source roots to the classpath when running GWT (using the same logic for ignoring source roots that are also declared as resources directories), and introduce a new gwt:package-lib goal (replacing jar:jar in the gwt-lib lifecycle) to package them in the JAR, without copying them to target/classes.
• localWorkers now accepts values as multipliers of the number of CPU cores, similar to mvn -T and Surefire's forkCount: e.g. 2 will use 2 workers, 0.5C will use as many workers as half the number of CPU cores.
• Remove default values from most GWT configuration properties: logLevel, failOnError, optimize, style, and localWorkers; only breaking change should be with localWorkers, previous behavior can be brought back using <localWorkers>1C</localWorkers>.
• annotation processing was actually only fixed for gwt-app, now fixed for gwt-lib as well.
• update Surefire to 2.20; this has an impact on gwt:test.