Doc: modularize Qt Designer custom plugin docs

Doc: modularize Qt Designer custom plugin docs

Create index and module pages for Qt Designer custom
plugin classes. Modify the qdocconf file to also build
the Qt Designer plugin class docs. Move snippets from
qtdoc to qttools. Fix links to the classes page in example docs.

Change-Id: Ie191fdb07fb0b00379a15d44c1355f00815053e6
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: David Schulz <david.schulz@digia.com>