Export/Import partial data

[noahjutz]

In order to share routines/exercises/workouts/programs (#51, #52) to other users, exporting and importing only selected data rather than the entire database must be implemented.

The planned UX:

• Upon pressing "backup" in the settings, a dialog shows up prompting the user to select which data to include/exclude (checkboxes for routines/exercises/etc.)

• Respective screens (RoutineEditor/ExerciseEditor) will have a "share" button

• Upon pressing "import", no data is erased. Data is only added, granted it isn't duplicate data.

• There will be an additional option to erase all data in case a user wants to restore a backup.

The codebase:

• The current implementation only allows for replacing the database file with another.
• Maybe solving this issue will require a different type of serialization (?)

[mbnoimi]

I missed this feature a lot. Today my wife spent 30 min cleaning my database (I sent to her) because she uses different programs (routines groups) of my own while both of us nearly use the same exercises.

[noahjutz]

I'm sorry about that, I still plan on implementing this feature, but please be patient. Pull requests are naturally always welcome.