Remove `trnsysType` argument to block item and child classes

[zuckerruebe]

As a developer I want to get rid of, e.g., an argument trnsysType with value "HeatPump" to the HeatPump class in its __init__ method If we need string constants to identify our components (e.g. for dragging them into the diagram) they should be constant fields of the classes themselves.

[zuckerruebe]

Drag'n'drop actually the only reason for having these at all I think (and maybe serialization, but not if we're using our serialization framework). There must be a smarter way of ding the drag'n'drop, like passing a serialized factory with the drop event.