Delete the frame callback when destroying a window

Not destroying the possible frame callback may lead to a crash when
calling wl_display_dispatch(), besides leaking memory.

Change-Id: Ida9f04322112673f4c1a8f3ab5a85c7c9d489dd3
Reviewed-by: Jan Arne Petersen <jan.petersen@kdab.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>