Revise legal notice

For GUI and non-daemon console, the legal notice won't ask for user acceptance anymore and only
provide an OK button (or Enter key in console) from now on.
For daemon mode, qbt will print the legal notice and continue to run. It will also notify user
to use command line option `--confirm-legal-notice` to suppress the message. The message will be
printed on every start up unless user specify the command line option once.

PR #20080.

src/base/settingsstorage.h

@@ -109,6 +109,7 @@ public:
 
     void removeValue(const QString &key);
     bool hasKey(const QString &key) const;
+    bool isEmpty() const;
 
 public slots:
     bool save();