MacroExpander: Allow registering local expansions

This allows a MacroExpander also to describe an expansion it can
do which was restricted to the global VariableManager only.

The global is now just a thin (unneeded) wrapper about new "standard"
functionality.

Change-Id: Ida7ca70cf3d319eae4220ea8d12f3dd1c0d4042c
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/plugins/coreplugin/jsexpander.cpp

@@ -90,7 +90,7 @@ QString JsExpander::evaluate(const QString &expression, QString *errorMessage)
 JsExpander::JsExpander()
 {
     d = new Internal::JsExpanderPrivate;
-    VariableManager::registerPrefix("JS",
+    globalMacroExpander()->registerPrefix("JS",
         QCoreApplication::translate("Core::JsExpander",
                                     "Evaluate simple Javascript statements.\n"
                                     "The statements may not contain '{' nor '}' characters."),