Add "Copy to Clapboard" to the export window

[ljubica-milovic]

Background
Many times, especially for "simpler" and linear formats like SMILES, FASTA, and HELM, the user does not need a file to be created, but rather the information to be copied to the clipboard so that they can paste it somewhere.

Requirements

1. In both molecules mode and macromolecules mode, a button should be added to the export window - "Copy to Clipboard".
2. When clicked, all the information that would get exported to a file gets copied to the clipboard.

UX
TBD