KNSWidgets::Action add compatibility for unified entry class in KF6
This way consumers can utilize the KNSCore::Entry class name already, which will be the unified entry class in KF6.
To avoid introducing an overloaded signal or a signal under a different name, the signal using the new class name is only visible when building KNewStuff or building with excluded deprecated API on the consumer side.
Relates to https://phabricator.kde.org/T15141
cc @osolovyov, since I did changes to the newly introduced KNSWidgets::Action class.