Move message filtering to C++

The filter callback is called very often (O(messages)). The current
filter model shows some significant overhead in QML internals. Moving
that to C++ makes it quite a bit faster.

src/CMakeLists.txt

@@ -6,6 +6,7 @@ add_executable(neochat
     clipboard.cpp
     matriximageprovider.cpp
     messageeventmodel.cpp
+    messagefiltermodel.cpp
     roomlistmodel.cpp
     neochatroom.cpp
     neochatuser.cpp