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/roommanager.h

@@ -13,7 +13,6 @@
 
 #include "chatdocumenthandler.h"
 #include "enums/messagecomponenttype.h"
-#include "eventhandler.h"
 #include "models/mediamessagefiltermodel.h"
 #include "models/messagefiltermodel.h"
 #include "models/roomlistmodel.h"