Remove dependency of the SVG icon engine plugin on QtWidgets.

Use the virtual QGuiApplicationPrivate::applyQIconStyleHelper()
introduced by commit 6c06e14a49773ce5572935864ed6b9be219c6103
in qtbase to adapt the icon according to the style.

Change-Id: I7c429f818c012c58118c64cc3b20a87fcb712877
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>