Improve binding coverage.

This change adds new bindings for the following GTK classes:

- GtkAboutDialog
- GtkAlignment
- GtkArrow
- GtkCalendar
- GtkDrawingArea
- GtkEventBox
- GtkSpinner
- GtkSwitch

In addition, new methods have been added for GtkLabel.

Initial work by @dajohi, cleaned up by @jrick.