SearchContext: do not emit the highlight-updated signal

When the search was implemented in gedit, there was the
search-highlight-updated signal, similar to the highlight-updated signal
from GtkSourceBuffer. So when moving the search implementation in
GtkSourceView, I reused the highlight-updated signal.

I think the signal is no longer required for the search. I added a
comment for the highlight-updated signal for explaining when this signal
is useful (and currently used in the spell gedit plugin).

Moreover, the search didn't send the signal at the right time. When
g_signal_emit_by_name() was called, the start and end variables could be
the entire buffer, resulting in performances problems when the spell
gedit plugin was enabled with the Highlight Misspelled Words option
(because the plugin connects to the signal, and rescan the specified
region).

https://bugzilla.gnome.org/show_bug.cgi?id=722535
View: do not connect to the highlight-updated signal

This code was added in the big commit "towards gedit integration" back
in 2003, without a comment explaining why this code was required.

I think it is no longer required. If a GtkTextTag is added to the
buffer, the view will normally be redrawn. And if a GtkTextTag is
already present in the buffer, but one of its property changes, the view
will normally also be redrawn.

It was perhaps used for updating the gutter, the right margin, etc. But
it seems to work fine without calling gtk_widget_queue_draw_area().

If something breaks, we can revert this commit, and add a comment
explaining why it is useful.