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>
Merge remote-tracking branch 'origin/release' into stable

Change-Id: I9c98f8277a31aa84e447262ae6745180880d4d2c