Write some formular widget

[Hu1buerger]

The formular widget should:

• show the option currently being edited
• a summary of all previous steps
• and should move to next step once the current step has been completed

[Hu1buerger]

This is partly addressed in 65fe69b2 with the add_stock widget which is partly evolving to a staged formular.

A staged formular is a formula that evolves in stages. First it builds stage 0, consecutively it will advance to stage 1...