Where possible, use version 1.6 for our classfiles.

--- a/openide.util/nbproject/project.properties
+++ b/openide.util/nbproject/project.properties
@@ -42,7 +42,7 @@
 
 javac.compilerargs=-Xlint -Xlint:-serial -Xlint:-processing
 javac.source=1.6
-javac.target=1.7
+javac.target=1.6
 module.jar.dir=lib