* new containment layout strategy for non-PanelContainments: lay them out in a lose gridish manner; not strictly a grid, and not strictly a packing-for-efficiency system, though it should be ok in that respect.

* allow containments to define what the layout should be

* a few less assumptions about panels and other containment types in the layout seting in addApplet

these are steps towards:
* more friendly for arbitrary containments on the desktop
* multiple containments
* ignoring braindamage that may eminate from the direction of x.org regarding screen geometries

we can now add and remove containments on the fly as well as lay them out nicely.

next: UI! and that's one more 4.1 task DONE.