glib-demo: Add annotations interactively

* Prepare UI to add multiple annotations type.
* Remove dialog and button add annotations.
glib: Add basic support for line annotations
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-demo: Add simple demo to add line annotations
glib-demo: Use a generic color chooser for new annotations

Set the color before adding an annotation instead of assigning
one by default and allowing a later modification.
glib-demo: Move the annotation's rectangle to properties

This allows to make the list view of annotations cleaner
glib: Add getter and setter of ending style to Line annotations
glib-demo: set ending styles when creating lines
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.
glib-demo: Add Square and Circle annotations demo
glib: Add PopplerQuadrilateral boxed type
glib: Add PopplerAnnotTextMarkup class and subtypes

The subtypes are: Highlihght, Squiggly, StrikeOut and Underline.
It adds convenient methods to set/get/free quadrilaterals
necessaries for TextMarkup annotations.
glib-demo: Add support for PopplerTextAnnotMarkup
glib-demo: Add demo for markup text annotations (WIP)
glib-demo: Add text markup annotations (highlight)