Introduce koin, add mockk for unit tests

[carstenhag]

• Introduce koin. For that, App.kt was added, which is a singleton class that subclasses Android's Application. In there, koin gets set up.
• Interfaces were added to classes that we want to be able to replace (usually for testing, but there's also a compose case at ExportImportDialog).
• If you want to use a class (or context) inside another class, define its constructor like this. Koin will then automatically pick a IPeriodDatabaseHelper etc:

class CalculationsHelper(
    private val dbHelper: IPeriodDatabaseHelper,
) : ICalculationsHelper {

• All of this means, that you should not create new instances of classes/interfaces like PeriodDatabaseHelper(). Class instantiation is only done by the DI framework, so here koin.
• Inside composables, we are for now using koinInject() to be able to use calculation/db classes. But this still breaks previews, as in there, koin is not actually in use. It compiles, but that's it 😛

In the future, we will have ViewModels. Those will then access IPeriodDatabaseHelper, IPeriodPrediction, etc.

Fixes #179, prerequisite for #97

[carstenhag]

@EmmaTellblom I removed this, felt like it was not really needed :)