Update info

[Hirschfuchs]

On new updates, there should be an info dialogue, what is new.

[Hirschfuchs]

User preferences (#26) are needed, because the dialogue should be shown only once on every update, except the user wants to keep seeing it

Tri-State: if not set, there are 2 Buttons: "do not show again" and "show again on next startup". If chosen first, it gets only shown again on the next update. If the second chosen, there will be a "Close" button the next time and a smaller checkbox for "show again", which is active by default.
Maybe also an option for "dont show at all", but that can be more hidden in the settings :)