Qt "Incompatible sender/receiver arguments" warning

The library view issues some warnings:

qt.core.qobject.connect: QObject::connect: Incompatible sender/receiver arguments
  QListWidget::currentRowChanged(int) --> plug::Library::load_slot(std::size_t)
qt.core.qobject.connect: QObject::connect: Incompatible sender/receiver arguments
  QListWidget::currentRowChanged(int) --> plug::Library::load_file(std::size_t)

gh#26