qt: handle `QVariant` in `MainCtx::urlListToMimeData()`

Sometimes the array elements may be variant (QVariant), even though the actual type is a QUrl or a QString.

This is an odd case because QUrl is automatically converted to url value type when passed to QML. However, the source url may already be provided as a QVariant, where in that case this conversion would not occur.

Request review @chub.