Sets the default mouse pointer for a Window.
Note that cursor must be for the same display as this.
Use Cursor.for_display or
Cursor.from_pixbuf to create the cursor. To make the cursor
invisible, use gdk_blank_cursor. Passing null for the cursor
argument to set_cursor means that this will use the cursor of its
parent window. Most windows should use this default.
| this |
a Window |
| cursor |
a cursor |