Create a content provider instance to get message content models from

This means that all content models will now come from the same source to remove duplication across multiple models and `chatbarcaches`.

It also handily breaks the dependency on needing `MessageContentModel` for `NeochatRoom`

src/CMakeLists.txt

@@ -198,6 +198,8 @@ add_library(neochat STATIC
     models/commonroomsmodel.h
     models/pollanswermodel.cpp
     models/pollanswermodel.h
+    contentprovider.cpp
+    contentprovider.h
 )
 
 set_source_files_properties(qml/TextToSpeechWrapper.qml PROPERTIES