Notify user if their kde-builder installation becomes outdated

With this feature we can ensure users have latest features of kde-builder.

The idea is the following. KDE Builder checks its current commit id, checks remote master commit id. If they are different, prints a warning message.

The second run does not show such message to not annoy. Next check is done only after a week.

This is how a warning message looks when outdated version detected:

Now, no another checks are made during a week.

If the user have the most recent version, once a week they will see such message:

If user do not want these checks at all, they can use check-self-updates: false in their config, or use --no-check-self-updates in the command line.

Tagging some people: @thiagosueto @merritt @fernando @tcanabrava @traceyc