Rebrand to Bitcoin Core

Only messages for now, executable names and other file names
can be changed later if necessary and safe.

Do not do an all-sweeping change. Some occurences of Bitcoin-Qt need to
be kept:

- Applicationname: this is used to determine the registry entry names,
  we don't want to lose settings over a silly name change.
- Where it refers to the executable name instead of the product name.

src/bitcoind.cpp

@@ -78,7 +78,7 @@ bool AppInit(int argc, char* argv[])
         if (mapArgs.count("-?") || mapArgs.count("--help"))
         {
             // First part of help message is specific to bitcoind / RPC client
-            std::string strUsage = _("Bitcoin version") + " " + FormatFullVersion() + "\n\n" +
+            std::string strUsage = _("Bitcoin Core Daemon") + " " + _("version") + " " + FormatFullVersion() + "\n\n" +
                 _("Usage:") + "\n" +
                   "  bitcoind [options]                     " + _("Start Bitcoin server") + "\n" +
                 _("Usage (deprecated, use bitcoin-cli):") + "\n" +