The JDK was always picked automatically (JAVA_HOME, PATH, the usual install dirs, by the
module's Java version) and Maven was the project's mvnw or mvn on PATH, with no way to
override either. languages.java.jdk (Languages & Frameworks › Java › Project JDK) now sets
the default jdtls runtime, the JDK of Run/Debug and JAVA_HOME for Maven and Gradle;
languages.java.maven (Build Tools › Maven) sets the Maven home, -s and
-Dmaven.repo.local for sync, builds, tool-window goals and detected run configurations.
JDKs and Maven installations are detected for the pickers.
Closes#13