Collage extensions and fixes

[s-dinda]

Prerequisites checklist

• I have read the contributing guidelines.

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

• Documentation update
• Bug fix
• New feature
• Other, please explain:

What changes did you make? (Give an overview)

• Added an option to set a placeholder (eg custom image / graphic) for one image of the collage.
• Collage Images will be dynamically scaled based on a given resolution (eg 300dpi etc)

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

• Please have a look at the collage creation process, including the admin options to select resolution and placeholder.

[andi34]

Hey and thanks a lot for your contribution!
I've picked your changes already to https://github.com/andi34/photobooth/tree/s-dinda/collage-extension for local testing and need to adjust 2 things. Trying to do later and push from https://github.com/andi34/photobooth/tree/s-dinda/collage-extension to dev branch once it's ready. Would that be ok? From Android development I am used to rebase changes before merge /submit to avoid conflicts by accident and to prevent a big history of "merge branch y into x" and so on 😅🙈

[s-dinda]

Hi!
Thanks a lot for your effort. Best Photobox!
Sounds great, so I guess I should close the PR?

[andi34]

Thanks a lot! I'll close this PR once ive finished it. I'll add a link to this PR inside the changelog.

[andi34]

b314760

fc73a42

c3dbd53

[andi34]

Rebased and merged! Thanks a lot!