Add functionality to the data grid for importing invoices

[timo2o1o]

Is your feature request related to a problem? Please describe.
As a user I would like to have some formatting of the values that gets shown in the data grid for importing invoices. For example the Text boxes displaying decimals shall be formatted accordingly. Also should the cells that are only informational and don't get evaluated by the migration be uneditable by the user. This helps to keep the confusion at a Minimum.

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