textbuffer: emit notify signal for the "text" property

textbuffer: emit notify signal for the "text" property

Although there is the "changed" signal, it is more correct to notify the
"text" property too. It can be useful for a small text view, where the
text is saved e.g. to gsettings with a binding to the text property.

The "text" property includes only the text, not child widgets or images,
so the notify signal is sent too many times (also for child widgets and
images), but it's not a big problem.

https://bugzilla.gnome.org/show_bug.cgi?id=624791