VCS[vcs,p4,svn,hg]: Use VCManager for delete operations.

...and its dialog prompt consistently. Rename method and
introduce overload taking IVersionControl. Add convenience slot to
VCSBasePlugin.

src/plugins/mercurial/mercurialcontrol.h

@@ -66,7 +66,6 @@ public slots:
 
 private:
     MercurialClient *mercurialClient;
-    bool mercurialEnabled;
 };
 
 } //namespace Internal