VCS[git]: Add support for stashes.

Add non-modal stash management dialog and additional menu option
"Stash snapshot..." to stash away changes prompting for a description,
which will immediately replay the stash (take snapshot and continue
working).
Add interface to IVersionControl for creating/restoring/deleting
snapshots for backup/complex undo operations (currently supported
by git only). Add test options to VCSBasePlugin.
Clean up and extend git client accordingly.

src/plugins/mercurial/mercurialcontrol.cpp

@@ -68,6 +68,7 @@ bool MercurialControl::supportsOperation(Operation operation) const
     case Core::IVersionControl::CreateRepositoryOperation:
         break;
     case Core::IVersionControl::OpenOperation:
+    case Core::IVersionControl::SnapshotOperations:
         supported = false;
         break;
     }
@@ -97,6 +98,26 @@ bool MercurialControl::vcsCreateRepository(const QString &directory)
     return mercurialClient->createRepositorySync(directory);
 }
 
+QString MercurialControl::vcsCreateSnapshot(const QString &)
+{
+    return QString();
+}
+
+QStringList MercurialControl::vcsSnapshots(const QString &)
+{
+    return QStringList();
+}
+
+bool MercurialControl::vcsRestoreSnapshot(const QString &, const QString &)
+{
+    return false;
+}
+
+bool MercurialControl::vcsRemoveSnapshot(const QString &, const QString &)
+{
+    return false;
+}
+
 bool MercurialControl::sccManaged(const QString &filename)
 {
     const QFileInfo fi(filename);