libtaskmanager: Use Remove/Insert to update the manual sort map

Otherwise the launcher items can't be dragged until the manual sort map
is reset after porting to QConcatenateTablesProxyModel, becuase in
TasksModel there is no slot for modelReset, and in KConcatenateRowsProxyModel,
after receiving a modelReset, there will be a rowsAboutToBeInserted signal,
but not in QConcatenateTablesProxyModel.