Linux-SonivoxEas v1.4.0

In this release the license has been upgraded to GPLv3. The sonivox library sources have been removed from the source tree, and replaced by a git submodule when the external library dependency is not found at configuration time. This is only for git users cloning the project from GitHub, because the released tarballs do not include the submodule.

There is also a new USE_QT option to choose between Qt5 and Qt6 when both major versions are present at configuration time. Qt6 is still experimental, while Qt5 is preferred. Finally, the build system now installs the icon and a desktop file.