This is a stop-gap patch solution for the annoying problem of models
being bad at updating. At least the process table will retain your
selection in SystemMonitor now.
AbstractView does not know which column it's displaying which makes it
impossible to implement the select_all functionality up there. Now
descendants override the pure virtual select_all method and implement
it themselves.
You can now drop things on an AbstractView, which will ask its model if
the drag is acceptable to drop at the index where it's dropped.
If it's accepted by the model, the view will fire the on_drop hook.
