Better Highlights: Rework highlight parsing order (#1524)

This commit is in response to #1523.

Whispers are now only added to the `/mentions` tab if they also match a
user name or phrase highlight. On a related note, the `highlightVisual_`
member has been removed as it is no longer necessary.

src/providers/twitch/TwitchMessageBuilder.hpp

@@ -92,7 +92,6 @@ private:
 
     const bool action_ = false;
 
-    bool highlightVisual_ = false;
     bool highlightAlert_ = false;
     bool highlightSound_ = false;