Don't return COW types by-pointer

These functions should return by-value, not by-pointer:

• QLabel::pixmap()
• QLabel::picture()
• QBrush::gradient() [*EDIT*: QGradient is not implicitly-shared]
• QCursor::bitmap()
• QCursor::mask()

The old API design is from the days of Qt 1, where the return types were not implicitly-shared. However, this is no longer the case.

Here's an abandoned initial attempt: https://codereview.qt-project.org/#/c/101233/10//ALL