Enable buttons for current game when receiving server response (#4737)

* Enable buttons for current game when receiving server response

Previously, upon joining a game, we were unconditionally re-enabling the
"Join" button in the lobby, even if it was not enabled in the first
place, causing #4698. This could also lead to issues where if the user
selects a different game after joining (which they can do in case of
e.g. network connectivity issues), the "Join as spectator" button could
get incorrectly disabled.

This fixes #4698 by re-enabling the buttons based on the state of the
currently selected game at the time the response is received. This also
avoids inconsistencies if a different game has been selected in between
joining and receiving a response from the server.

* Typo: enable gameButton in enableButtons

The "create game" button was incorrectly being disabled in enableButtons
whereas (as the name indicates) it should have been enabled

* Remove misleading comment about race conditions

cockatrice/src/gameselector.h

@@ -49,6 +49,9 @@ private:
     GameTypeMap gameTypeMap;
 
     void updateTitle();
+    void disableButtons();
+    void enableButtons();
+    void enableButtonsForIndex(const QModelIndex &current);
 
 public:
     GameSelector(AbstractClient *_client,