windeployqt: Plugin dependencies aren't properly checked

windeployqt appears to copy all plugins in iconengines and imageformats without checking their dependencies. For example, the qsvg icon engine and image format plugins get deployed, but QtSvg does not. There may be other plugins that have the same problem.

This actually causes store certification to fail on WinRT due to missing linkage. In any case, it causes useless bloat (the plugins won't work without their dependencies anyway).