gtklinkbutton: Set cursor on button's event window

gtklinkbutton: Set cursor on button's event window

Setting the cursor on the widget window (i.e. the parent widget's) is
finicky because the cursor needs to be updated on crossing events, and
will yield the wrong result for other master devices that happen to be
in other areas of the same parent widget's window.

Just set it always on the event window created by the GtkButton parent
class. That window was causing the crossing events, so the rectangle
that gets the hand cursor set will be the same size, and we don't need
to track pointer crossing state that way.

https://bugzilla.gnome.org/show_bug.cgi?id=785375