Doc: Rename Qt Designer license information page

Rename the license information page to a more generic one, and list
licenses for individual contributions under it.

Task-number: QTBUG-28536
Change-Id: Ia88d99640ab23837f4db2a820fc95c9318d3b365
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>