Add metadata structure for F-Droid

[Korb]

it is strongly encouraged to add metadata in the application’s source repo for for the following reasons:

1. The application’s metadata is under direct control of the repository owners
2. Metadata from the repository is copied to the F-Droid repo automatically (thus no merge requests are needed)
3. It is the only way to provide images, such as the app icon or screenshots

In F-Droid client, since release 1.6, only applications that have all of the following are shown on the Latest tab:

1. Name
2. Icon
3. Summary
4. Description
5. License
6. a What’s New entry for at least one release
7. at least one graphic (screenshot or a feature graphic)
8. at least one of the above translated

Source — https://f-droid.org/en/docs/All_About_Descriptions_Graphics_and_Screenshots/

Currently, TimerDroid’ metadata is taken from the F-Droid database (with all the limitations that entails). In particular, I would like to translate full_description.txt for languages ​​I know well and add screenshots in those languages ​​so that it is obvious to users that the app has been translated into their language.