introducing the PositioningRuler widget:
it's a qwidgets that features a wordprocessor like ruler that can be used to reposition and resize the panel.
its elements are drawn by Plasma::Svg, so also a proper svg (widgets/containments-controls) is added.
it mostly works but still needs some love

* the not-left alignments are still not tested
* the panel still don't save its minimum and maximum sizes