[Panel] Allow arbitrary panel placements

(autarkper: only apply changes to some files, out of dalcde's big patch)
(autarkper: with the necessary adapations to make it fit into my model)
[Window Manager] When window-list applet (or equivalent) is not found, use "scale" minimize effect instead of "traditional")

(autarkper: with some refactoring)
[panels] Show MOVE_DROP icon when dragging over panel.
[panels] Position applet tooltips without assuming so much about the current monitor configuration.
[panels] Populate panel menu only on demand.
[panels] Set autohide on/off directly in panel menu.
[panels] Move box-positioning responsibility around.
[panels] Add set-function for panel's hideable-flag.
[panels] Simpler, more descriptive panel-layout names

"Traditional" -> "Bottom"
"Flipped" -> "Top"
"Classic" -> "Top + Bottom"
"Classic Flipped" -> "Bottom + Top"