Improve screenshots layout

I tried to improve the screenshot layout in terms of usability and design:

• Few screenshots that don't fill the screens full width will now be displayed centered (see "Nextcloud Dev" as example app).
• Screenshots will be displayed bigger than before. The size matches roughly that of Google Play. This way the user can see at least a little bit of the screenshots contents.
• All screenshots have a small margin so multiple screens can be distinguished. A nice side effect is that the elevation shadow will not be cropped any longer.
• I removed the highlight (elevation and larger size) from snapped screenshots. At first I wanted to highlight the centered screenshot during scrolling instead (see #1068 (closed)) but there is no easy way to do this. I think removing the highlight results in a much smoother user experience as it did not add any benefit for the user and felt really "jumpy".

Before and after the changes:

Please let me know what you think and if I can improve anything.