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/qt/guiutil.cpp

@@ -594,7 +594,7 @@ void restoreWindowGeometry(const QString& strSetting, const QSize& defaultSize,
 HelpMessageBox::HelpMessageBox(QWidget *parent) :
     QMessageBox(parent)
 {
-    header = tr("Bitcoin-Qt") + " " + tr("version") + " " +
+    header = tr("Bitcoin Core") + tr("version") + " " +
         QString::fromStdString(FormatFullVersion()) + "\n\n" +
         tr("Usage:") + "\n" +
         "  bitcoin-qt [" + tr("command-line options") + "]                     " + "\n";
@@ -607,7 +607,7 @@ HelpMessageBox::HelpMessageBox(QWidget *parent) :
         "  -splash                " + tr("Show splash screen on startup (default: 1)") + "\n" +
         "  -choosedatadir         " + tr("Choose data directory on startup (default: 0)") + "\n";
 
-    setWindowTitle(tr("Bitcoin-Qt"));
+    setWindowTitle(tr("Bitcoin Core"));
     setTextFormat(Qt::PlainText);
     // setMinimumWidth is ignored for QMessageBox so put in non-breaking spaces to make it wider.
     setText(header + QString(QChar(0x2003)).repeated(50));