UI: Make Picture/Camera screens fit device

[cyberme0w]

Hi! This PR is related to #17 - it takes users a bit of time to realize you need to scroll to see the Take Picture/Save Picture buttons. Here's what I did:

• Replaced SingleChildScrollView with a Container. This avoids having to scroll to see components, and allows us to have a safety margin around the container, preventing components from touching the screen's edges.
• Used Expanded and Flexible to dynamically resize the components (the button takes up as much space as it needs first, and the rest is given to the preview)
• Added a SizedBox between the camera preview and the screen's respective button to avoid them touching.

Preview on phone:

Preview on tablet:

I think Flutter has a way of adding overlays to the camera controller, which would allow for a prettier full-screen camera, but I'm not familiar with it, and I think for the time being, this could be a good alternative.

Please let me know if you have any suggestions or comments. I'd be happy to help :)

[abertschi]

Thanks for the PR, I will give you a review on this in the near future. :+1: