UI: squared design, action buttons inside gallery

[andi34]

Prerequisites checklist

• [x] I have read the contributing guidelines.

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

• [x] Allow taking pictures from gallery
• [x] Other, please explain: new design

What changes did you make? (Give an overview)

Adapted new style by @Moarqi and adjusted for Photobooth integration to be optional available via admin panel. Adapted changes for action buttons inside gallery, made available where needed.

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

[andi34]

While on it, To-Do:

• rounded edges inside gallery for modern theme
• add color changes from @Moarqi and make squared the default theme for next release?