x-compile kotlin/java: unify and improve compiler plugin setup

[jangalinski]

Can we get rid of this redundant declaration of src java/kotlin:

 <plugin>
          <!-- Kotlin compiler -->
          <artifactId>kotlin-maven-plugin</artifactId>
          <groupId>org.jetbrains.kotlin</groupId>

          <executions>
            <execution>
              <id>compile</id>
              <goals>
                <goal>compile</goal>
              </goals>
              <configuration>
                <sourceDirs>
                  <sourceDir>${project.basedir}/src/main/kotlin</sourceDir>
                  <sourceDir>${project.basedir}/src/main/java</sourceDir>
                </sourceDirs>
              </configuration>
            </execution>
            <execution>
              <id>test-compile</id>
              <goals>
                <goal>test-compile</goal>
              </goals>
              <configuration>
                <sourceDirs>
                  <sourceDir>${project.basedir}/src/test/kotlin</sourceDir>
                  <sourceDir>${project.basedir}/src/test/java</sourceDir>
                </sourceDirs>
              </configuration>
            </execution>
          </executions>
        </plugin>