Change color of annotation window

When the color of the annotation window is chosen
in the properties dialog and apply is clicked, the
color of the window changes immediately.
Fixed by making ev_annotation_set_rgba send both
"color" and "rgba" signals.

https://bugzilla.gnome.org/show_bug.cgi?id=725571
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