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/guiconstants.h

@@ -2,7 +2,7 @@
 #define GUICONSTANTS_H
 
 /* Milliseconds between model updates */
-static const int MODEL_UPDATE_DELAY = 500;
+static const int MODEL_UPDATE_DELAY = 250;
 
 /* AskPassphraseDialog -- Maximum passphrase length */
 static const int MAX_PASSPHRASE_SIZE = 1024;