Add 2x icons for retina displays.

This change updates the mode selector/fancyactionbar
in Qt Creator with new high-resolution icons.

Set Qt::AA_UseHighDpiImages on Qt 5.1 and higher to
make QIcon generate high-dpi pixmaps. Add "@2x" mode
images.

Update StyleHelper::drawIconWithShadow to handle high-dpi
pixmaps. The shadow-drawing algorithm is kept in device
pixels.

Change-Id: I411b7a24e534a2d75a1bbdc4d10219dcbea26bed
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

src/app/main.cpp

@@ -307,6 +307,10 @@ int main(int argc, char **argv)
     QtSystemExceptionHandler systemExceptionHandler;
 #endif
 
+#if QT_VERSION >= 0x050100
+    app.setAttribute(Qt::AA_UseHighDpiPixmaps);
+#endif
+
     // Manually determine -settingspath command line option
     // We can't use the regular way of the plugin manager, because that needs to parse pluginspecs
     // but the settings path can influence which plugins are enabled