License change proposal to address https://github.com/randrew/phantom…

[twardoch]

This pull request implements the original author’s intention to allow this package to be used by holders of the commercial Qt License. This is basically done the same way as The Qt Company does it (or specifically: used to do it around Qt 5.3, when they published dual-licensed LGPL 2.1+commercial versions). This is to address the original author’s note: https://github.com/randrew/phantomstyle#license