- Introduce Qt 5 signal-slot connection syntax.
- Merge MainWindow::createMenus()/createActions()
  into MainWindow::createMenus(), removing the need
  to store the actions as member variables.
  Use QMenu::addAction() for brevity.
- For actions in QActionGroups, carry the Valuator enum
  in QAction::data so that the slot to handle the selection
  does not need to compare the QAction pointer itself.
- Use a non-modal QColorDialog, so that the user
  can change colors more easily while drawing.
- Choose saner shortcut keys: control-Q should not
  override the default usage for quitting the application,
  and using shortcuts for About dialogs is anyway dubious.
- Improve the example documentation.

Change-Id: I57aaf5f5b885c13a953482dbcc41275dd3d6bff4
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@theqtcompany.com>