Change the opacity of the annotation window

Implements the functions needed to change the
opacity of the annotation windows.

https://bugzilla.gnome.org/show_bug.cgi?id=725571
Fixing annotation window colors

The annotation window now is all of one color and
no border.
Improving annotation window appearance

The title is now bold and there is extra margin
space around the textbox.
Also fixing minor typo.
Closing annotation window by clicking on icon

When the annotation window is opened, clicking on
the corresponding icon closes it.
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
Modifying popup menu for annotations

The popup menu that appears with a right-click on
an annotation icon contains only annotation
options ('properties' and 'delete').
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
Implementing delete function for the popup menu

Completed the implementation so that when 'delete'
is clicked, the annotation is actually deleted.
Saving annotations when the window is focused

When an annotation is edited and the user clicks
the top right 'x' to close evince's main window
while the annotation window itseld still has the
focus, the changes on the annotation are
acknowledged and the user is prompted to save
them.