NotifyBlockTip signal: switch from hash (uint256) to CBlockIndex*

- also adds a boolean for indication if the tip update was happening during initial sync
- emit notification also during initial sync

src/main.cpp

@@ -2636,9 +2636,11 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
             // Notify external listeners about the new tip.
             if (!vHashes.empty()) {
                 GetMainSignals().UpdatedBlockTip(pindexNewTip);
-                uiInterface.NotifyBlockTip(vHashes.front());
             }
         }
+        if (!vHashes.empty()) {
+            uiInterface.NotifyBlockTip(fInitialDownload, pindexNewTip);
+        }
     } while(pindexMostWork != chainActive.Tip());
     CheckBlockIndex(chainparams.GetConsensus());