Make Qt5 static plugins work

With introduction of QPA every single Qt app has at least one dynamically loaded plugin the one in 'platforms' folder.

This breaks Qt static build (ot compiles, but the application fails with exception).

To make static builds work I hacked qplatformintegrationfactory_qpa.cpp to return QWindowsIntegration instance explicitly,
then link windowsd.lib into final app. Also qRegisterWidgetsVariant() method had to be called explicitly.

An option of configure script similar to qt-sql<driver> should be introduced (e.g. qt-qpa<driver>) to handle this case in a nice way.