Switch to compose state and kotlin flows - 2. iteration

[sunkup]

Purpose

Replacing LiveData with compose state and flows, will in the future hopefully prevent the issues that came with LiveData. Also it makes for easier integration with DAVx5.

Short description

This is the 2. Iteration of the switch from LiveData to compose state and kotlin flows. See the 1. iteration here.

The 1. Iteration took care of the Database Access Objects. It deprecated LiveData returning methods and added their respective Flow returning variants.

In this 2. iteration models and activities are rewritten, by replacing all LiveData usages in both models and activities, excluding any 1. iteration DAO changes, since this PR would grow too big otherwise.

The 3. iteration will take care of using the new Flow returning methods in DAOs and remove the last remaining LiveData usages.

This PR:

• Prefers compose state over StateFlows where possible
• Uses StateFlow in CredentialsModel and SubscriptionSettingsModel since their flow type members are merged in EditCalendarActivity and it is not possible to merge compose states, to my knowledge. In any case, the declaration of modelsDirty needs special attention.
• introduces uiState data classes with private set where possible

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[ArnyminerZ]

I have not commented on everything, but as stated, use Flow or Compose states as:

var variable by mutableStateOf(...)
  private set

And then in the composable, just use =:

val variable = model.variable

And set all values from the viewmodels through setters as needed.

[sunkup]

I think this should be it for now? Should turn a lot prettier with the 3rd and 4th iterations.

[ArnyminerZ]

Just one state missing 😉 (see unresolved conversation)