QTableView: add getter and setter for cornerbutton

QTableView has a function cornerButtonEnabled as well as cornerWidget (resp. the setter function). The cornerWidget function is inherited from QScrollArea and has nothing to do with the cornerButton in QTableView. Since we wanted to modify the behaviour of the corner button (QTableView), we found the cornerWidget, but had to learn that the cornerWidget of QScrollArea is not the same as the cornerButton of QTableView and that there is no reasonable way to obtain the cornerButton of QTableView.

So we suggest two things:
1. Better names to distinguish between the two corner buttons.
2. Getter (and maybe setter) for the QTableView corner button.