Licence in about dialog GPL 2, not GPL 2 or later

#### Bug Summary

The About LMMS dialog box and LICENSE.txt state "GPL 2" and do not mention or later. This would be incompatible with "GPL 3" as used by external code and QT. I feel this is just an oversight as a quick grep of the code base shows "GPL 2 or later"

#### Affected LMMS versions

Only checked 1.2.2

Inspect the About LMMS dialog and LICENSE.txt, then compare their wording with the existing “GPL 2 or later” references found by grep. Done means both the dialog and license file state the intended GPL wording and the affected license text is consistent.

Written by the indexing model from the issue text.