textview: add extend-selection signal

To be able to customize the double-click and triple-click behaviors, to
provide other selection boundaries.

Note that the GtkTextExtendSelection enum contains DOUBLE_CLICK and
TRIPLE_CLICK, not WORDS and LINES, so totally different boundaries can
be provided, it is not restricted to words for double-click and lines
for triple-click. For example a triple-click on some C code could select
the current scope.

https://bugzilla.gnome.org/show_bug.cgi?id=111503