Expose form fields as AtkObject children of the page

https://bugzilla.gnome.org/show_bug.cgi?id=728475
Setting parent for annotation properties dialog

After the last gtk+ update, the warning:

"Gtk-Message: GtkDialog mapped without a transient
parent. This is discouraged."

was shown whenever the annotation properties
dialog was opened. The commit fixes this issue by
setting this dialog transient for the main window.
As a plus, the dialog is now shown always on the
center of the main window.

https://bugzilla.gnome.org/show_bug.cgi?id=732114
libdocument: Add remove_annotation to DocumentAnnotations.

https://bugzilla.gnome.org/show_bug.cgi?id=649044
libview: add ev_view_remove_annotation

Hide annotation windows if necessary and unset selected
element before removing an annotation.

https://bugzilla.gnome.org/show_bug.cgi?id=649044
Add Delete Annotation option to the view popup

Based on this patch:
https://bug649044.bugzilla-attachments.gnome.org/attachment.cgi?id=275731
by Jose Aliste.
Adapted to the new menu.
libdocument: add ev_mapping_list_set_remove

Needed to remove annotations from a mapping list

https://bugzilla.gnome.org/show_bug.cgi?id=649044
pdf: Implement remove_annotation virtual func

https://bugzilla.gnome.org/show_bug.cgi?id=649044
Refreshing annotations' sidebar upon deletion

When an annotation is deleted, the sidebar needs to
be refreshed so that the annotation is removed from
the list

See item 3 in this comment:
https://bugzilla.gnome.org/show_bug.cgi?id=649044#c33