gutterrendererlines: avoid extra function calls in ::query_data()

We can remove the calls to check if the cursor is visible for every visible
line by keeping track of when the property changes. This removes a
gtk_text_view_get_cursor_visible() and gtk_text_gutter_render_get_view()
for every line in the rendering process.

https://bugzilla.gnome.org/show_bug.cgi?id=765356