Use source indication in activate window

Bug: 10747

xfce4-panel was not setting the source indication
for NET_ACTIVE_WINDOW messages so we could not use it.

Bug 10508 got that fiex in xfce4-panel so we can (and must)
use the source indication.

Note, this will break with earlier (broken) xfce4-panel
versions.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
Adjust shadows

Use a thinner shadow with light source from the top
on default theme, slightly larger for hdpi and
xhdpi variants.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>