Handle Qt 5 platform plugins
Qt 5 seems to introduce a new concept of platform plugins, dynamically loaded to integrate the program with the operating system. So on Windows it needs a
platforms\qwindows.dll plugin for the application to work at all. The abstraction is briefly documented here.
This should be simple to handle automatically - we just need a hook that will check which platform it's on and copy the relevant plugin file to the target directory. We need to find out whether QtCore requires the platform plugin, or if it's only when QtGui is in use.
Prompted by this StackOverflow question.