Bitcoin-Qt: save and restore position of debug window

- move the code for saving and restoring window positions from BitcoinGUI
  to GUIUtil, make it more generic and also use it for saving/restoring
  debug window positions

src/qt/guiutil.cpp

@@ -24,6 +24,8 @@
 #include <QFileDialog>
 #include <QDesktopServices>
 #include <QThread>
+#include <QSettings>
+#include <QDesktopWidget>
 
 #include <boost/filesystem.hpp>
 #include <boost/filesystem/fstream.hpp>
@@ -487,6 +489,29 @@ bool SetStartOnSystemStartup(bool fAutoStart) { return false; }
 
 #endif
 
+void saveWindowGeometry(const QString& strSetting, QWidget *parent)
+{
+    QSettings settings;
+    settings.setValue(strSetting + "Pos", parent->pos());
+    settings.setValue(strSetting + "Size", parent->size());
+}
+
+void restoreWindowGeometry(const QString& strSetting, const QSize& defaultSize, QWidget *parent)
+{
+    QSettings settings;
+    QPoint pos = settings.value(strSetting + "Pos").toPoint();
+    QSize size = settings.value(strSetting + "Size", defaultSize).toSize();
+
+    if (!pos.x() && !pos.y()) {
+        QRect screen = QApplication::desktop()->screenGeometry();
+        pos.setX((screen.width() - size.width()) / 2);
+        pos.setY((screen.height() - size.height()) / 2);
+    }
+
+    parent->resize(size);
+    parent->move(pos);
+}
+
 HelpMessageBox::HelpMessageBox(QWidget *parent) :
     QMessageBox(parent)
 {