Allow browsing for multiple service types simultaneously

[hrzlgnm]

• sync browsed service-types between backend and fronend
  • event browser-started (service_type)
  • event browser-stopped (service_type)
• allow for starting browsing for not yet browsed service-types
• add stop button for each running service-type browse
• background event sending thread is started once, separately
• add separate button for clearing the discovered services

[hrzlgnm]

We may need to track the receivers from ServiceDaemon.browse in the MdnsState in the backend and asynchronously receive events from them. Remove the receivers at SearchStopped of their service_type.