Vcs: Move handling of settings from VcsBaseClient to VcsBaseClientImpl

... and update users of that functionality accordingly.

Unexpected plus: Now every supported VCS actually saves their setting
when requested.

Change-Id: I02db7b2ce14e5f52d26409b2a01aea290c2a294a
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/mercurial/mercurialcontrol.h

@@ -70,7 +70,6 @@ public slots:
     // files changed signals according to the variant's type:
     // String -> repository, StringList -> files
     void changed(const QVariant&);
-    void emitConfigurationChanged();
 
 private:
     MercurialClient *mercurialClient;