Use RichText text input to provide real mentions

This use internally now a QTextDocument and a new C++ class to
manipulate the document with QTextCursor.

src/CMakeLists.txt

@@ -15,6 +15,7 @@ add_executable(neochat
     main.cpp
     notificationsmanager.cpp
     sortfilterroomlistmodel.cpp
+    chatdocumenthandler.cpp
     ../res.qrc
 )