Peptide list is not reset when creating a new assay

[pverscha]

When a new assay is created from a list of peptides, the input field is not reset for the next time that a peptide list needs to be inputted.

Steps to reproduce:

• Create a new assay from a list of peptides
• Open the dialog to create a new assay from peptides
• The list is not empty anymore. It should be empty by default.

[pverscha]

This has been fixed in #104