Added adapter factory for Form editors to integrate to VIATRA tooling

Note: xtend-maven-plugin version was bumped because of
https://github.com/eclipse/xtext/issues/1231



Change-Id: Iba6b5dffad144430f896e36fe873cd84c439c7be
Signed-off-by: Balázs Grill <balazs.grill@incquerylabs.com>