Revert "use Xtext 2.7.0 nightly"

This reverts commit 26d553cfa947efae042693d1457acbb3e10207e8.

Conflicts:
	plugins/it.xsemantics.dsl/src/it/xsemantics/dsl/jvmmodel/XsemanticsJvmModelInferrer.xtend
	releng/it.xsemantics.parent/pom.xml
	tests/it.xsemantics.dsl.tests/src/it/xsemantics/dsl/tests/generator/XsemanticsJvmModelGeneratorTest.xtend
removed dep on version 1.2.1 of Xcore

Change-Id: I81a281f807ba7026778c70c2bb509f3563a7bdb3
Revert "30: use new StandardTypeReferenceOwner"

This reverts commit 8e9e6784e699c9441e1425a7a1aad480019370a7.
Reverted to Xtext 2.6 inferrer API

Change-Id: I9f0dc3670a5993ba03387829e541e3c35bee56b9
readapted tests to Xtext 2.6

Change-Id: I83afe863b406dfdfd137d8f789ee3f27af2add2c
removed dep on org.eclipse.xtend.lib.macro

Change-Id: Ia06bf8dc598222725218e7328a5c92a63962a80d
reverted TP to Xtext 2.6.2

Change-Id: If86260c29305691b0ad8b986ab22e73c8743dd06
Updated content assist test for Xtext 2.6

Change-Id: I0733477a435df7496493efe3bc11b675c2a63277
refer to single release number for download.itemis p2 repository

Change-Id: I33cc558f671369bb58739fd76ac2a6e0463c2f38
version 1.6.x for Xtext 2.6 in category.xml

Change-Id: I1585ccf47a4ceb817c18fa727705328213710aae
updates.dir -> backports for composite repo

Change-Id: I8b6989a654aa6caaefe819daa4f40b42ac3cc503