Clarify what is "recent activity" in our room list ordering settings

As seen in the bug report, this setting is a bit confusing. First, it
refers to "message activity" but in reality it does take into account
all events. This is fine in my opinion, so I clarified that point.

Another thing is that it wasn't clear that timeline visibility settings
currently affect the sorting, so I added a tip about that.

Finally I wasn't happy with these two options being called "Activity"
so the old "Activity" setting is now called "Importance". The "Last
Message Activity" setting is now called "Newest Events".

BUG: 508480
FIXED-IN: 25.12.0

src/libneochat/enums/roomsortparameter.h

@@ -89,7 +89,7 @@ public:
         case Parameter::MostHighlights:
             return i18nc("@info", "Rooms with the most highlighted messages are higher");
         case Parameter::LastActive:
-            return i18nc("@info", "Rooms with the newer messages are higher");
+            return i18nc("@info", "Rooms with newer events are higher");
         default:
             return {};
         }