Store tasks and products in `pd.DataFrame` objects.

[eike-welk]

Tasks and products are currently stored in trivial classes without methods. Therefore they could also be stored as rows in pd.DataFrames. Some code could be deleted if all important pieces of data would be stored in the same way.

There are also complex requirement to access tasks and products, that are more easily fulfilled by pd.DataFrame:

• Access by numerical index, as well as string ID.
• Access objects for which a certain condition is true.

Therefore research if this is possible. Important issues:

• Inserting a row into the list view at an arbitrary position.
• Deleting rows.

  Actions

• Create a unified QAbstractTableModel for listings and prices. Use this class to predict if exclusive use of pd.DataFrame would lead to more elegant solutions.
• Design a unified XML storage solution for listings and prices.

[eike-welk]

This will be done in the new software.