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/debugger/debuggerplugin.cpp

@@ -3334,7 +3334,7 @@ bool boolSetting(int code)
 QString stringSetting(int code)
 {
     QString raw = theDebuggerCore->m_debuggerSettings->item(code)->value().toString();
-    return VariableManager::expandedString(raw);
+    return globalMacroExpander()->expandedString(raw);
 }
 
 QStringList stringListSetting(int code)