Open-source installer license description text needs to be updated for Qt 5.7.0

The description text in the beginning of open-source license text inside Qt 5.7 installer needs to be updated to reflect the licensing changes in Qt 5.7.

Correct text could be, for example: "...licenses Qt under open source licenses, with most of the functionality available under LGPLv3 license. It should be noted that the tools as well as some add-on components are available only under GPLv3."

There also is no need to have LGPLv21 text at the end of the license page any more, as the only use of LGPLv21 is in the 3rd party licensed parts of Qt WebEngine. There is already text referring to the 3rd party component licenses, where LGPLv21 is included.