Small GUI improvments

[lukas-monet]

User story

1. As a user
2. I want a userfriendly and nice looking user interface
3. So that I can use the software easily

Acceptance criteria

• Space diffscreen column should be evenly and not bottom aligned (in the video difference screen)
• Settings button & Navigation Buttons should be replaced by Icon Buttons with Fixed size (like the full screen buttons / appbar buttons) with background color (in the video selection screen)

Definition of done (DoD)

• Create feature branch
• Implement the functionality
• Opened pull request and assigned reviewer
• If necessary: required changes are implemented
• Delete feature branch
• If necessary: Component Test has been written
• Code review has been completed and code has been merged

DoD general criteria

• Feature has been fully implemented
• Feature has been merged into the mainline
• All acceptance criteria were met
• Product owner approved features
• All tests are passing
• Developers agreed to release