Apply line-numbers style in GtkSourceGutter

The line-numbers style was applied to the whole GtkTextView widget with
gtk_widget_override_color() and gtk_widget_override_background_color().
It worked because the 'text' style is also applied to the main
GtkTextView region with CSS, which seems to override the colors set with
gtk_widget_override_color() and gtk_widget_override_background_color().

Now the line-numbers style is applied directly in GtkSourceGutter, which
is more logical.

It permits to provide a 'line-numbers' style without providing the
'text' style. The 'text' style is not provided by the classic and tango
style schemes, but a different style for 'line-numbers' is desirable.

https://bugzilla.gnome.org/show_bug.cgi?id=733606