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/node/interface_ui.h

@@ -105,7 +105,7 @@ public:
     ADD_SIGNALS_DECL_WRAPPER(NotifyBlockTip, void, SynchronizationState, const CBlockIndex*);
 
     /** Best header has changed */
-    ADD_SIGNALS_DECL_WRAPPER(NotifyHeaderTip, void, SynchronizationState, const CBlockIndex*);
+    ADD_SIGNALS_DECL_WRAPPER(NotifyHeaderTip, void, SynchronizationState, int64_t height, int64_t timestamp, bool presync);
 
     /** Banlist did change. */
     ADD_SIGNALS_DECL_WRAPPER(BannedListChanged, void, void);