Various improvements on media-service-demo

I've done some changes in media-service-demo:
* player widget layout changes
* fixed seeking
* rewrote the SearchModel as a ListStore: this is simpler and less error prone
* fixed search so it re-queries if there's more data

I'm fine with it if you want to deal with the repo-restructure before this. Just sent it now so you have the choice. Also: some of the changes are such that I would normally have discussed them with others before doing them, but as I considered this more a learning experience I thought it doesn't really matter if some improvements aren't accepted: it'll still be useful for me.

Any general comments are also welcome: like should I use topic branches and a pull request per feature etc.

I believe this fixes issues #4, #5 and #6 which I just found.