Improve JoinRoom Server List

Initial work to create a model so that new servers can be added to the list.
The model will also check if the server is valid before allowing it to be added.

Implements network/neochat#11

### TODO
- [x] Add functionality to cache added servers

src/CMakeLists.txt

@@ -40,6 +40,7 @@ add_library(neochat STATIC
     completionmodel.cpp
     completionproxymodel.cpp
     actionsmodel.cpp
+    serverlistmodel.cpp
 )
 
 add_executable(neochat-app