a11y: compute properly if there is text selected

https://bugzilla.gnome.org/show_bug.cgi?id=722188
clutter-text: emitting ClutterText::delete-text before actual changes on the text

https://bugzilla.gnome.org/show_bug.cgi?id=722220
(cherry picked from commit bbc7d20f5ecff80f4000557b976e3153f6286252)
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
clutter-text: emitting ClutterText::insert-text before actual changes on the text

https://bugzilla.gnome.org/show_bug.cgi?id=722220
(cherry picked from commit cadbeceff0a729cdeafd84db237ed44ad782e820)
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>