Introduce gtk_render_insertion_cursor

The new function provides an API that takes the PangoLayout and index
as input params, this way it handles strong and weak cursors internally
factoring out all code duplicated in the widgets that need to render
cursors.

https://bugzilla.gnome.org/show_bug.cgi?id=640317

docs/reference/gtk/gtk3-sections.txt

@@ -5738,6 +5738,7 @@ gtk_render_slider
 gtk_render_activity
 gtk_render_icon_pixbuf
 gtk_render_icon
+gtk_render_insertion_cursor
 
 <SUBSECTION Standard>
 GTK_TYPE_STYLE_CONTEXT