F53: Updates introduction screen to fit new features and design.

See merge request !84

F51 image rights

Closes #87

See merge request !82

# Conflicts:
#	lib/lang/app_de.arb
#	lib/ui/screens/detail_screen.dart
#	lib/ui/screens/loading_screen.dart
#	lib/ui/widgets/artwork_card.dart

# Conflicts:
#	lib/api/model_cache.dart
#	lib/ui/screens/detail_screen.dart
#	lib/ui/screens/exhibitions_screen.dart
#	lib/ui/widgets/artwork_card.dart
#	lib/ui/widgets/carousel_slider_campus.dart
#	lib/ui/widgets/carousel_slider_exhibition.dart

Some small hotfixes

Closes #91

See merge request !83

- deleted unnecessary code
- added some comments

# Conflicts:
#	lib/api/model_cache.dart
#	lib/ui/screens/detail_screen.dart
#	lib/ui/widgets/image_text_overlay.dart

Fixes small bug in the loading screen

- When the smartphone loses the internet connection a screen is displayed which tells the user to reconnect to the internet
- No more red screens are displayed. Instead the images that could not be loaded are presented as a red exclamation mark.

Changed the checkmark icon in the color settings

Updated the map compass icon
Co-authored-by: egallouis <egallouis@users.noreply.github.com>

Co-authored-by: egallouis <egallouis@users.noreply.github.com>