Return Result for errors

[EPashkin]

For throwable function return value must return Result (see discussion beginning at #117 (comment)).

2 types of throwable functions:

1. without other outs that will returns Result<(), glib::Error>
   1.1) Examples of FFI function return guint:
   gtk_builder_add_objects_from_string
   1.2) Examples of FFI function return gboolean:
   gtk_css_provider_load_from_path,
   gtk_file_chooser_add_shortcut_folder,
   gtk_file_chooser_set_current_folder_file,
   gtk_page_setup_load_file,
   gtk_print_settings_to_file
   1.3) Examples of FFI functions return something usable
   gtk_icon_info_load_icon,
   gtk_recent_info_create_app_info,
   gtk_recent_manager_lookup_item
2. with other outs that will returns tuple(or single value) instead () on good case
   Examples:
   gtk_builder_value_from_string,
   gtk_icon_info_load_symbolic

Blocked by #103

[gkoz]

The 1) would probably be Result<ActualReturnType, glib::Error>

Except, if a throwing function returns Boolean, it can just be ignored making it Result<(), glib::Error>.

A list of throwing functions, return types and annotations: https://gist.github.com/gkoz/17f2ad8a05447b235b27

[EPashkin]

You right about return value.
Thanks for list.

[gkoz]

Error has conversions impls now, solving this will bring us pretty close to complete coverage of existing gtk API.

[EPashkin]

Good, I try do it tomorrow