webkitgtk: new version 2.6.1, pin version 2.4.6 to webkitgtk_2_4

  with version 2.6.1, support for WebKit1 API has been removed.

  now we have:
    webkitgtk     -> webkit2 for gtk3
    webkitgtk2    -> webkit1 for gtk2
    webkitgtk_2_4 -> webkit1/webkit2 for gtk3