Saner defaults for GWT compilation (#12653)

 * persistentunitcache and other irrelevant stuff now guided to target directory
    where it don't end up growing war files
 * disable compile report (aka soyc) by default, for most widget set compilations this is obsolete

Change-Id: Ia6d908517e9a35e5ffd14e13bbd37c26e44bfdad