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/mercurialclient.h

@@ -42,9 +42,7 @@ class MercurialClient : public VcsBase::VcsBaseClient
 {
     Q_OBJECT
 public:
-    MercurialClient(MercurialSettings *settings);
-
-    MercurialSettings *settings() const;
+    MercurialClient();
 
     bool synchronousClone(const QString &workingDir,
                           const QString &srcLocation,