Plasma/libplasma2/migratetoqtcomponents

Discussion

QML allows faster UI creation compared to old widgets, Scene-graph doesn't support QGraphicsWidget

Proposed Solution

Gradual change: along with the development of QtCompoents, a Plasma-themed component set should be done, will hopefully go into use before libplasma2, so plasmoids can migrate to them before libplasma2, easing the port. The whole QGraphicsWidget (and QGraphicsProxyWidget) based set currently in libplasma should be dropped (since won't work in Scene-graph) so the replacement has to be as complete as possible. It should also significantly decrease the size of the library.