Rework event handler to be just a series of static helper functions

- Clear out unused functions
- All functions are now static

This is because we pretty much always used it in the form:
```
EventHandler eventHandler(room, event);
eventHandler.function();
```
This simplifies it all to a single call.

src/models/roomtreemodel.cpp

@@ -357,8 +357,7 @@ QVariant RoomTreeModel::data(const QModelIndex &index, int role) const
         if (room->lastEvent() == nullptr || room->lastEventIsSpoiler()) {
             return QString();
         }
-        EventHandler eventHandler(room, room->lastEvent());
-        return eventHandler.subtitleText();
+        return EventHandler::subtitleText(room, room->lastEvent());
     }
     if (role == AvatarImageRole) {
         return room->avatar(128);