Doc: Drag.mimeData QVariantMap is documented as "stringlist"

Wrong documented type of Drag::mimeData. It is implemented as QVariantMap (which is a type alias to QMap<QString, QVariant>), but the property is documented to be of a nonexistent superficial type "stringlist".

I don't know how to document that type properly, but even the more correct one list<string> is not suitable here.

Besides, code examples use the notation of an inline JavaScript object. While it works in this particular case, in other parts of QML documentation it was stated that such braces may (and will) be treated as block of code, leading to syntax errors. It is suggested to use braces inside parenthesis instead, like this:

Drag.mimeData: ({prop: val})