Display an image of the invoice the user is working on

[timo2o1o]

Is your feature request related to a problem? Please describe.
As a user I want to see an image of the invoice I'm currently working on. That helps me to insert all nescessary information.

Describe the solution you'd like
Using the linked document, we can localize the path of an invoice and display the pdf-file in a browser element.

Acceptance criteria

• The invoice shall be displayed on the following windows:
  • Import and edit outgoing invoices
  • Import and edit incoming invoices

Additional context
Add any other context or screenshots about the feature request here.