Improve notifications

Currently implemented actions:

• Open (default)
• Mark as read

Issues on Android:

• The indexes of the actions seem to differ from those on Linux; Maybe that should be fixed in KNotifications
  • The index of the first action button is 0 instead of 1
  • The default action (the notification itself) does not have an index and no signal is emitted for it
  • The latest version on the master branch of KNotifications contains an additional signal for the default action
    • Verify that signal is emitted
• The notification is not removed after clicking on it
• The app stays in the background after clicking on the notification

General issues:

• The display name of the chat partner is not shown anymore as the title of the chat page because of necessary refactoring work