Rework downloading into Midori.Download interface

Rework downloading into Midori.Download interface

Midori.URI
    get_folder
Midori.Download
    is_finished
    get/set_type
    get_progress
    get_tooltip
      Consistent estimation of remaining time and speed.
    get_content_type
      Icons in dialog and panel.
    has_wrong_checksum
      Verification in both panel and toolbar.
    action_clear, action_stock_id, open
      Consistent behavior in both panel and toolbar.
    fallback_extension
    clean_filename
        Merged two separate functions.
    get_suggested_filename
    get_filename_suggestion_for_uri
        Now handles MIME types.
    get_extension_for_uri
    get_unique_filename
        Renamed from prepare_filename.
    prepare_destination_uri
    has_enough_space