Add more notification settings

[nucleus-ffm]

• implement improved notification settings. It is now possible to select for which source and severity you want to receive a notification
  • will hopefully be a workaround for Too many notifications #91 and pre 0.4.1 Feedback: warning overflow #51
• fix a bug of an error whereby a method was executed too often

[MatsG23]

I think we should only show the AlertSwiss source in the notification settings if it has been turned on in the general settings.
Now that we have a settings page for the notifications, would it not make sense to also move the "show status notification" settings there?

[MatsG23]

It would be also a cool addition if we could hide (collapse) the frequency of background updates setting when background updates are disabled in general.

[MatsG23]

Let me explain my quite big changes to the code for the notification source settings:

• storing a Severity instead of NotificationLevel in the NotificationPreferences (removal of NotificationLevel enum)

• adding a disabled property to NotificationProperties
  => makes re-enabling restore the last notification level for that source

• removal of Severity.other (as it is has always been treated as Severity.minor)

• make filling the NotificationProperties with initial data a loop and move this to the class with the default values for the app settings
  => as the app already needs these settings when it calls Place#_checkIfEventShouldBeNotified (used to have unprecise checks until you open the notification settings page)

• reversing the order of severities on the slider (Severity.minor is now 0 etc.)

Instead of the NotificationLevel enum that had values like "upToMinor" (specified a range), the app now checks the enum index value of the stored notification level (a Severity) and the warning. This is a much simpler approach.

[MatsG23]

TODO: Also replace convertSourceToInt (services/sortWarnings.dart) with a method in the related enum class (use a for-loop as seen in enums/Severity.dart in the getIndexFromSeverity method for simplicity)
=> check the order of the enum values (especially LHP, AlertSwiss and Other) as they had no unique integer before (convert function returned 0 for everything except Mowas, Katwarn, Biwapp and DWD - although this likely aims for a stable order for all providers - see sortWarnings function)