Enable macro editing for the Clang indexer

Refactor much of the code from Environment* classes to NameValue* classes
to share it with the preprocessor macro settings.

Change-Id: Ica4ee817aa338230c422b30d91240d266248d226
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmlprojectmanager/fileformat/qmlprojectitem.cpp

@@ -98,7 +98,7 @@ bool QmlProjectItem::matchesFile(const QString &filePath) const
     return false;
 }
 
-QList<Utils::EnvironmentItem> QmlProjectItem::environment() const
+Utils::EnvironmentItems QmlProjectItem::environment() const
 {
     return m_environment;
 }