glib: Add implementation of Square and Circle annotations

Square and Circle only differ in the constructor which defines
the subtype.  Therefore, it uses the same name than Poppler's
Geometry class.

https://bugs.freedesktop.org/show_bug.cgi?id=70983
glib-demo: Add Square and Circle annotations demo

https://bugs.freedesktop.org/show_bug.cgi?id=70983