gui2/taddon_connect: Add a Help button to this dialog, shows the same page as the Add-ons Manager's own Help button

Thanks to lipkab for help with the buttons layout trick.

The rest is kind of a hack because:

  1) GUI2 doesn't use display objects, so I needed to acquire one from
     the constructor (optionally, to avoid problems with the test suite)
  2) If the button id is "help", it is handled by a hardcoded hack that
     handles the titlescreen dialog in gui2::twindow::get_retval_by_id()
     and closes the dialog prematurely right after our local callback is
     executed; so the id is "show_help" instead

Notice that there is precedent for (1) in gui2::teditor_settings.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56175 75b3775d-3801-0410-9e12-e2e43121521d