Move the plugin howto from qtdoc to corelib where
it belongs. Fix the snippets and remove all
remaining references to Q_EXPORT_PLUGIN
Task-number: QTBUG-26237
Change-Id: I43dce2ffa42193b7a992fa1a0f2fcb2f633037b2
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>