Variables: Use lambdas as callbacks to resolve variables

Instead of broadcasting for each resolution we ask the code
that knows about a specific variable directly.

Change-Id: I2f0f4f2acceba85a236995d236980594a3166bd8
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/vcsbase/vcsplugin.h

@@ -72,7 +72,6 @@ signals:
 
 private slots:
     void slotSettingsChanged();
-    void updateVariable(const QByteArray &variable);
 
 private:
     void populateNickNameModel();