Fix Twitch-Specific Filters Not Being Applied

[Nerixyz]

Description

#4107 previously tried to fix this issue, but was later reverted (#4111), because having a filter like channel.live in a live channel would "freeze" (deadlock) the app (#4110). This PR fixes this deadlock and applies the fix by #4107.

The deadlock was caused, because the lock-guard in TwitchChannel::setLive was kept alive for too long. Specifically, setLive adds a message which then gets to the filter which tries to access streamStatus_, which is locked in setLive (deadlock).

I prevented this by reducing the critical-section where the guard is kept alive.

I also noticed that TwitchChanne::isLive called access over accessConst which lead me down a shallow rabbit-hole:

• The UniqueAccess for streamStatus_ isn't actually needed (probably similar for others as well). If you add assertInGuiThread before every access, you'll see that the app works fine. This is because (1) helix-calls aren't concurrent, (2) command-variables are substituted concurrently, (3) commands aren't executed concurrently, and (4) filters aren't checked concurrently.
• TwitchChannel::twitchBadge, TwitchChannel::cheerEmote, TwitchBadges::badge, DebugCount::getDebugText, and ChannelChatters::colorsSize use UniqueAccess::access over UniqueAccess::accessConst, even though they don't modify the underlying data.
• UniqueAccess<T> &operator=(const T &element) and UniqueAccess<T> &operator=(T &&element) are unsound. They should lock the mutex before modifying the data. Only UniqueAccess<T> &operator=(const T &element) is used here.

Fixes #4112.

[pajlada]

Thanks for looking into this, good fix 👍