Fix connect() calls inside plugins

Previously there were lots of messages like:

QObject::connect: Cannot connect (null)::ProcessedResourceEvents(EventList) to StatsPlugin::addEvents(EventList)
QObject::connect: Cannot connect (null)::RegisteredResourceMimetype(QString, QString) to StatsPlugin::saveResourceMimetype(QString, QString)
QObject::connect: Cannot connect (null)::RegisteredResourceTitle(QString, QString) to StatsPlugin::saveResourceTitle(QString, QString)
QObject::connect: Cannot connect (null)::pluginConfigChanged() to StatsPlugin::loadConfiguration()

They were all attempting to connect to a null pointer because every
plugin is a separate copy of the Module::Private::s_instance QHash

Fixed by using a shared library that all plugins link against