Simplify ConversationOverview code for sections

[FelberMartin]

Problem

The code for section handling in the ConversationOverview and ConversationList is currently quite messy. There is no central place that defines a section and its properties, instead we have different variables and methods all over the place:

• UI
  • Keys for each section in ConversationList
  • Key suffixes in ConversationList
  • A separate method for toggling each section in the ConversationOverviewViewModel
• Other
  • A variable for most of the sections in ConversationCollections
  • ConversationPreferenceService.Preferences with again one variable for each section
  • Keys for storing the preferences in ConversationPreferenceStorageServiceImpl

This issue is not strictly blocked, but I suggest to wait until #259 is merged.

Suggested changes

• At a central place, create a data model for a section, containing all necassary variables:
  • Key for UI and Storage
  • Key suffixes for the conversations listed in this section
  • ... (Whatever else is needed)
• This new model may also replace the need for ConversationCollections and allow for cleaner code in places this is currently used