glib: Add getter and setter for annotation's rectangle

Annotation objects contain at least two keys, Rect and Subtype.
The former has the coordinates where the annotation is placed.
The getter and setter allows to obtain and modify the position
of a given annotation.

https://bugs.freedesktop.org/show_bug.cgi?id=70901
glib-demo: Add annotations interactively

* Prepare UI to add multiple annotations type.
* Remove dialog and button add annotations.

https://bugs.freedesktop.org/show_bug.cgi?id=69978