Unify projectexplorer language IDs

We don't need two IDs for C++ and the QmlJS ID should look the same as
as the others.

Change-Id: Ib9747f6b36a90bb652951d85eec69666615670c4
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cppeditor/cppeditor.cpp

@@ -104,7 +104,7 @@ namespace Internal {
 
 CppEditor::CppEditor()
 {
-    addContext(ProjectExplorer::Constants::LANG_CXX);
+    addContext(ProjectExplorer::Constants::CXX_LANGUAGE_ID);
 }
 
 class CppEditorWidgetPrivate