gtklinkbutton: Set cursor on button's event window

gtklinkbutton: Set cursor on button's event window

It is not necessary to (re)set the cursor on every crossing
event, and can probably yield the wrong results if there are
multiple master devices involved. Just set it on init(), and
let the inner machinery update the cursor whenever necessary.

This patch is an adaption of commit 0daf79676 in gtk-3-22, the
side effects are not as bad here because the cursor was already
being set on the widget specifically instead of the parent
widget's, but there's still some nonetheless (plus, it's simpler)

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