Change progress bar from block-based to time-based

This is less confusing to most people, and doesn't rely on estimates
of the total number of blocks received from other nodes.

src/qt/bitcoingui.h

@@ -98,6 +98,8 @@ private:
     RPCConsole *rpcConsole;
 
     QMovie *syncIconMovie;
+    /** Keep track of previous number of blocks, to detect progress */
+    int prevBlocks;
 
     /** Create the main UI actions. */
     void createActions();