Support non-deletable activities (again)

[tfiedlerdev]

Description

When we introduced the use case concept (#161 ) we needed to get rid of default labels (activities and their categories), since they were hardcoded in the source code and did not fit into our generic concept.
Back then the hardcoded default categories and activities were added as non-deletable items to the dialog.
Since we do not have hardcoded categories and activities anymore, we can't add them as non-deletable.
However we could store the "deletability" of an activity/category inside the json and thereby add them accordingly. (might be useful for future data recording projects that use our app)

Must Include

• support non-deletable activities and categories by also storing this propoperty in the json

[tfiedlerdev]

This is being done with #162 . Closing this issue now