windeployqt feature for qmake .pro files

Adding CONFIG+=windeployqt to a project should add an automatic post-build deployment step that invokes windeployqt.

It could use QMAKE_EXTRA_COMPILERS as a post-build step to do the job, and have settings for controlling the command sent to windeployqt. The default invocation could be as simple as
windeployqt $$TARGET