implement countdown offset to compensate shutter-delay and cheese time

[Metropo]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "x" next to an item)

• New feature

What changes did you make? (Give an overview)

I noticed, that some users that wanted to take pictures with the photobooth seemed to be confused or misled on the time the picture is taken. They assumed that at the moment the box displays "Cheeeeeese" and the live preview hides the image gets taken. This is probably encouraged by the mirror of the DSLR going down between live view and taking the picture.
A colleague suggested changing the countdown in a way, that it reaches 0 at the approximate time the photo is really taken. I implemented it using an offset that is added to the displayed countdown. It looks like this now
6, 5, 4, 3, Cheeeeeese [1 second] + shutter delay + Autofocus [1-2 seconds]
In my opinion, it is more intuitive this way. I would be glad to hear your opinion.

Is there anything you'd like reviewers to focus on?

[andi34]

Thanks! I'll take a closer look at it later today. Changes to the language files only need to be done to the english file, all other languages need to be translated at crowdin once submitted ( https://crowdin.com/project/photobooth ).

[Metropo]

@andi34 Thanks for the information. I reverted the change to the german langauge file.

[Metropo]

I fixed the issues and rebased onto current dev branch.