signal editor: Avoid GTK+ warnings

GTK+ warns nowadays if the state passed to gtk_style_context_get_*
does not match the state of the context. Avoid this by setting
the state beforehand. This is just a bandaid fix; the drawing
should be redone in terms of gtk_render_ APIs instead of poking
at colors.

https://bugzilla.gnome.org/show_bug.cgi?id=758299