sprint-10-release-candidate

• setup UI-Tests by @simonsasse in #166
• remember last open folder in file picker by @simonsasse in #189
• restructor UI by @a-miscellaneous and @AlperK61 in #193
• add a short documentation of the algorithm by @simonsasse and @akriese in #173
• add a screen preview by @akriese and @AlperK61 in #122
• export of all inserted frames as a png list by @fs3itz in #174

Full Changelog: sprint-09-release...sprint-10-release-candidate