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 files copied over from kdesignerplugin.

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22785