Replace sqlite-net-pcl with something else

Since [sqlite-net-pcl](https://github.com/praeclarum/sqlite-net) is by far not well enough maintained we should switch away from it.

An alternative would be to use [Entity Framework Core](https://docs.microsoft.com/de-de/ef/core/).
There is just one problem with it:
* It requires `16299` as min SDK version but phones only support `15063` as min SDK. So we would have to use a version < 2.1.