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

Summary:
Enables the library to provide its own plugin, instead of having to refer
to the single plugin from KDesignerPlugin module, which pulls in
dependencies to all other covered KDE Frameworks modules.

Icon file copied over from kdesignerplugin.

Reviewers: Frameworks

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22790