qt: Revamp ClientModel code to handle {Block|Header}Tip core signals

No behavior change.

src/qt/clientmodel.h

@@ -23,6 +23,7 @@ enum class SynchronizationState;
 namespace interfaces {
 class Handler;
 class Node;
+struct BlockTip;
 }
 
 QT_BEGIN_NAMESPACE
@@ -104,6 +105,7 @@ private:
     //! A thread to interact with m_node asynchronously
     QThread* const m_thread;
 
+    void TipChanged(SynchronizationState sync_state, interfaces::BlockTip tip, double verification_progress, bool header);
     void subscribeToCoreSignals();
     void unsubscribeFromCoreSignals();