Feature/clear alerts on player change

[thomassedlmayer]

User-Facing Changes

Message converter alerts are cleared when opening a new source/player.

Description

I noticed that I forgot to clear alerts when opening a new source over an already open one in #943. Currently, any alerts that were emitted from previous message conversions are kept across opened sources. This adds ambiguity about which source lead to the alert. Message converters can't create source-aware alert IDs because they don't know anything about the source.

Since message converters are the only component that use session alerts right now, this change only affects alerts emitted by message converters.

Checklist

• The web version was tested and it is running ok
• The desktop version was tested and it is running ok
• This change is covered by unit tests
• Files constants.ts, types.ts and *.style.ts have been checked and relevant code snippets have been relocated