PhoshLayerSurface: Destroy layer surface on unmap
This and moving some of the initilization from realized
to mapped
allows us to properly gtk_widget_{show,hide}()
layer surface widgets.
This is brought over from squeekboard c3a54595ea72638fbc7b65ffc3c26966de772e15
Signed-off-by: Guido Günther guido.gunther@puri.sm