feat: better autocompletion UX

The autocompletion bar is now more similar to mainstream
chat applications, which use up/down to change the selection,
and tab to confirm the selection. An extra space is also added
to keep the flow of typing going.

src/chatdocumenthandler.cpp

@@ -197,7 +197,7 @@ void ChatDocumentHandler::replaceAutoComplete(const QString &word)
         }
     }
 
-    cursor.insertHtml(word);
+    cursor.insertHtml(word + " ");
     m_lastState = cursor.block().text();
     cursor.endEditBlock();
 }