[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