[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