glib-demo: Fix rectangle calculation for new annotations in demo

https://bugs.freedesktop.org/show_bug.cgi?id=69978
glib-demo: Rearrange layout for annotations in demo

Add render area to visualize the annotations per page.

https://bugs.freedesktop.org/show_bug.cgi?id=69978
glib-demo: Merge columns Type and Color in annotations demo

Reduce the space used by both columns, makes the color
pixbuf smaller enough to give a clue of the annotation
color.

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