Small optimization for a common case (no marks)

This function is called at every redraw, so it is worth catching this common case
Deprecate gtk_source_completion_move_window()

The window is already moved automatically with
gtk_source_completion_provider_get_start_iter().

When generating the documentation, there is a warning:
 warning: gtk_source_completion_move_window is deprecated in the inline
 comments, but no deprecation guards were found around
 the declaration. (See the --deprecated-guards option for
 gtkdoc-scan.)

But it's normal, it's a bug in gtk-doc (bug #624001):
https://bugzilla.gnome.org/show_bug.cgi?id=624001
Completion: fix the documentation for the page sizes

I thought that the provider-page-size property was not used. But one can
emit the "move-page" signal with @step equal to GTK_SCROLL_PAGES.
CompletionInfo: use gtk_bin_get_child()

Instead of storing the widget in the private structure, we can simply
use gtk_bin_get_child().
Deprecate gtk_source_completion_info_{get,set}_widget()

If one day the CompletionInfo becomes more complex (like it was in the
past, with a ScrolledWindow and a Viewport), it will still be possible
to override gtk_container_add() and gtk_bin_get_child().
Don't use gtk_source_completion_info_{get,set}_widget()

It's less convenient to use the GTK+ functions, but it's better to have
a smaller API.