Slide downloading saturates internet connection

With the new slide facility, when someone enters [SESSIONSTART] every active client goes to the web server and download the slides. With many viewers, and a restricted internet connection, this will quickly saturate the internet connection and will lead to severe delays before *anyone* gets a copy of the slides.

Two fixes could be implemented:

1) On the network the presentation is to be carried out on, anyone who already has a copy of the slides could advertise that they have the slides via ZeroConf, and either multicasted out to listening peers (in larger environments, using a link-local multicast address) or unicast transmitted to anyone who wants a copy.

2) Pre-loading of content. Registering your interest in a talk means that once the slides have been published, they are downloaded at a point before the talk begins. This could also tie into a feature that allows you to pick and choose which tracks you're interested in, and maintain a calendar that is shown in a Lernid "tab" or similar.

Of course, both would be preferable - with the fallback mechanism of just loading the slides when you start Lernid, and watching the topic etc for the latest slide number.