Introduce class ItemWithParticles as base for ParticleItem et. al. Make SessionGraphicsItem virtual base class.

Major change: use `SessionItem* parent` instead of `QModelIndex parent` in SessionModel::insertNewItem's input parameter list. This is done to avoid multiple calls item->index(), and make the interface more uniform. Code is not in a compilable state.

Major change: use `SessionItem* parent` instead of `QModelIndex parent` in SessionModel::insertItem's input parameter list. This is done to avoid multiple calls item->index() in about 300 places, and make the interface more uniform. Code is not in a compilable state.

As recommended by
https://google.github.io/styleguide/cppguide.html#The__define_Guard