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/projectexplorer/projectmacroexpander.cpp

@@ -99,7 +99,7 @@ bool ProjectMacroExpander::resolveMacro(const QString &name, QString *ret)
 {
     bool found = resolveProjectMacro(name, ret);
     if (!found) {
-        QString result = Core::VariableManager::value(name.toUtf8(), &found);
+        QString result = Core::globalMacroExpander()->value(name.toUtf8(), &found);
         if (ret)
             *ret = result;
     }