glib: Add basic support for line annotations
glib: Add simple demo to add line annotations
glib: Use a generic color chooser for new annotations demo

Set the color before adding an annotation instead of assigning
one by default and allowing a later modification.
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.

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
glib: Add getter and setter of ending style to Line annotations
glib: Move the annotation's rectangle to properties in demo

This allows to make the list view of annotations cleaner
glib: Add ending style for lines in demo
glib: set ending styles when creating lines in demo