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.cpp

@@ -38,8 +38,7 @@
 using namespace Mercurial::Internal;
 
 MercurialControl::MercurialControl(MercurialClient *client)
-        :   mercurialClient(client),
-            mercurialEnabled(true)
+        :   mercurialClient(client)
 {
 }