Avoid deprecation warnings 2: miscellaneous

Follows !21 (merged).

This MR handles various isolated deprecations within objects that are not deprecated themselves:

• gtk_message_dialog_set_image()
• g_settings_list_keys()
• gtk_source_view_set_draw_spaces()
• gtk_menu_popup()
• Miscellaneous
  • gtk_font_button_get_font_name()
  • gtk_window_set_has_resize_grip()
  • gtk_show_uri()

Except for the dialog boxes, where it was not possible for me to fully reproduce the appearance of a GtkMessageDialog, these changes should be invisible.