Issue #1022 - Adjusted Messaging API for SDK

This MR contains the following changes:

• app.get
• filters.get
• filters.getTypes
• filters.importRaw
  • Removed "removeExisting parameter".
    • Stopped exporting allowlistedDomainRegexp.
• filters.listen
  • Converted filter text argument of "filterState.enabled" event to Filter object.
  • Fixed: En-/disabling filters not reflected in custom filter table.
    • Removed "filters.listen" message in io-filter-list.js, because it was ignored. Instead, added "changed" to "filters.listen" message in desktop-options.js.
    • Stopped using filterNotifier for listening to filter state changes, in favor of filterState.
• filters.remove
• subscriptions.get (see also commit)
• subscriptions.listen
  • Listen to multiple filterNotifier events to combine changes in a single "changed" event.
• Refactored populateLists() function in desktop-options.js to simplify the code that had to be changed to accomodate "filters.get" and "subscriptions.get" changes.
• Updated test environment accordingly.
• Updated documentation accordingly.