Add option to build Qt Designer plugin (BUILD_DESIGNERPLUGIN, default ON)

Summary:
Being a "Porting Aids", the KF tier3 kdesignerplugin can no longer
do the service of providing such a plugin, as this means depending on us.
So we serve ourselve here for those still using KDEWebKit and preferring
to have a proper representation when using Qt Designer.

Test Plan:
Building with BUILD_DESIGNERPLUGIN=ON generates a working plugin,
which appears in "Views (KDE - Deprecated)" group.
Still also builds with BUILD_DESIGNERPLUGIN=OFF

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22581