Make validation interface capable of signalling header presync

This makes a number of changes:
- Get rid of the verification_progress argument in the node interface
  NotifyHeaderTip (it was always 0.0).
- Instead of passing a CBlockIndex* in the UI interface's NotifyHeaderTip,
  send separate height, timestamp fields. This is becuase in headers presync,
  no actual CBlockIndex object is available.
- Add a bool presync argument to both of the above, to identify signals
  pertaining to the first headers sync phase.

src/qt/clientmodel.cpp

@@ -267,8 +267,8 @@ void ClientModel::subscribeToCoreSignals()
             TipChanged(sync_state, tip, verification_progress, /*header=*/false);
         });
     m_handler_notify_header_tip = m_node.handleNotifyHeaderTip(
-        [this](SynchronizationState sync_state, interfaces::BlockTip tip, double verification_progress) {
-            TipChanged(sync_state, tip, verification_progress, /*header=*/true);
+        [this](SynchronizationState sync_state, interfaces::BlockTip tip, bool presync) {
+            if (!presync) TipChanged(sync_state, tip, /*verification_progress=*/0.0, /*header=*/true);
         });
 }