Improve / clean some code

android/gradle.properties

fastlane/metadata/android/en-US/changelogs/50.txt

fastlane/metadata/android/fr-FR/changelogs/50.txt

lib/ui/widgets/abstracts/custom_bar_chart.dart

lib/ui/widgets/abstracts/custom_chart.dart

lib/ui/widgets/abstracts/custom_line_chart.dart

lib/ui/widgets/cards/counts_by_day.dart

lib/ui/widgets/cards/counts_by_hour.dart

lib/ui/widgets/cards/discoveries.dart

lib/ui/widgets/cards/heatmap.dart

lib/ui/widgets/cards/new_artists.dart

lib/ui/widgets/cards/new_tracks.dart

lib/ui/widgets/cards/statistics_global.dart

lib/ui/widgets/cards/statistics_recent.dart

lib/ui/widgets/cards/timeline.dart

lib/ui/widgets/cards/top_artists.dart

lib/ui/widgets/charts/counts_by_day.dart

lib/ui/widgets/charts/counts_by_hour.dart

lib/ui/widgets/charts/heatmap.dart

lib/ui/widgets/charts/timeline_eclecticism.dart