panel: create panel objects on correct screen panels when using separate x screens

This fixes bug: https://bugzilla.gnome.org/show_bug.cgi?id=704019

Signed-off-by: Philipp Kaluza <phk@src.gnome.org>
panel: fix an uninitialized variable warning
panel: fix deprecated warnings (part 1)
panel: fix deprecated warnings (part 2)
panel: fix gnome-panel background

Closes bug https://bugzilla.gnome.org/show_bug.cgi?id=704203