[Klipper] Handle incoming UTF-8 mime types
QMimeData abstracts most mime type handling, but not text/plain vs text/plain;charset=utf-8.
This is currently being done in QWayland inside qwaylanddataoffer. Given this code is effectively a clone, we need to have the same check here.
Interestingly mimeData->text() worked before but not hasText(), which is why it was unnoticed in the test pasteClient tool.
This fixes klipper and GTK apps.