Allow for pasting plain ("unstructured") text into any placeholder

[eugenschindler]

Currently, it is possible to paste plain text (intention or right-click action called "Paste Unstructured Text From Buffer") into the top-level collection of a TextgenText root. I think there are more often applied use cases where it makes sense to also paste plain text that will then be analyzed and converted to a plaintextgen collection into other elements, not only in the top-level element. Since the functionality for analyzing and pasting plain text as a plaintextgen collection (vertical, horizontal, spaceindented) is already there, we probably only need to allow the action/intention on other collections than the top-level collection.