add status icon & auto-check for updates

Hi Tim

Really nice work on the yumex application! I added a few extensions and like for you to pull them if you agree & like them.

These are the additions:
- the backend does not have to be started as root anymore. It will automatically restart if root priviledges are needed
  (currently only: applying pending actions, enable/disable repositories persistently, are there more?)
- a status icon which shows the number of available updates
- command line option: --start-hidden to start with a hidden main window (useful for autostart)
- automatically check for new updates with configurable interval

It is not finished yet, there are a few things that need to be done:
- translation of all added text (status icon, option dialog)
- for the icon I did not use the yumex app icon, because it looks really bad if it is small. Perhaps a modified version of it would be better.
- option to add yumex to autostart
- only lock yum if really needed instead of during the whole runtime

cheers
Beat