[Qt] misc small Mac related changes/cleanups

- cleanup Info.plist.in and specify high DPI mode enable command as per
  http://blog.qt.digia.com/blog/2013/04/25/retina-display-support-for-mac-os-ios-and-x11/
- move setting of QApplication::setAttribute() to bitcoin.cpp and add
  attribute for enabling use of high DPI pixmaps for Qt >= 5.1
- add missing setWindowTitle() on Mac
- cleanup Mac / non-Mac setup in bitcoingui.cpp

src/qt/bitcoin.cpp

@@ -200,6 +200,13 @@ int main(int argc, char *argv[])
 
     Q_INIT_RESOURCE(bitcoin);
     QApplication app(argc, argv);
+#if QT_VERSION > 0x050100
+    // Generate high-dpi pixmaps
+    QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
+#endif
+#ifdef Q_OS_MAC
+    QApplication::setAttribute(Qt::AA_DontShowIconsInMenus);
+#endif
 
     // Register meta types used for QMetaObject::invokeMethod
     qRegisterMetaType< bool* >();