Add ability to specify custom mouse pointer in QML

Qt Widgets appear to have the ability to specify an arbitrary mouse pointer, by setting the cursor property to a QCursor constructed from a QPixmap. This should be possible in a QML MouseArea, too, except that its cursorShape property is just an enumeration. One possibility would be to extend the enumeration statically, with Qt.User0Cursor through Qt.User9Cursor, and then provide a function for associating an image and hotspot with any of those values.