Small optimization for a common case (no marks)

This function is called at every redraw, so it is worth catching this common case
Rework GtkSourceMark binary search

Add SourceMark unit test that was failing with the old implementation
and rework the implementation to properly deal with multiple source
marks at the same iter.
Updated Lithuanian translation
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.