grapher: implement restarting a stap process

grapher: implement restarting a stap process

* grapher/StapParser.cxx (StapParser::disconnect): new function
* grapher/StapParser.hxx (StapProcess::StapProcess): initialize argv to 0
* grapher/grapher.cxx (StapLauncher::setArgs): Set argv to 0
  (StapLauncher launch, launchUsingParser): Refactor launch(),
  extracting function a that (re)launches a stap process using an
  existing parser.
  (StapLauncher::onChildDied): call disconnect() on dead parser.
  (GrapherWindow::_graphicalLauncher, setGraphicalLauncher): delete
  member, replacing with...
  (graphicalLauncher): new variable
  (ProcModelColumns): Store parser object in the list model instead of
  just a StapProcess object.
  (ProcWindow::onRestart): new function
  (ProcWindow::refresh): Preserve the list selection when the process
  list is refreshed.
  (ProcWindow::onSelectionChanged): Manage the restart button's state.