Add Copy to Clipboard button to export dialog

[lucas-koehler]

Motivation
Currently, the schema or ui schema need to be manually selected and copied in the export dialog. This is tedious for large (ui) schemas because Ctrl+A does not work for this (it selects too much). Consequently, the (ui) schema needs to be manually selected with the mouse.

A similar button is already present on the left where the (ui) schema can be edited. Thus, it seems inconsistent to not have it in the export dialog.

Solution
Add a 'Copy to Clipboard' button that copies the contents of the current tab to the clipboard.