Redesign Plasma to work with multiple processes (MP)

The big challenge

it requires sharing input events and coordinating shared rendering .. unlike web browser tabs, plasmoids are not completely separate but need to communicate at least with their host container. so there is a lot of message passing and shared data access that would be required. then we add the need for fluid animations involving multiple components at once.

what could be put into its own process is DataEngines, however. so MP is not entirely verboten, just not for the painting parts
MP also raises the memory usage *significantly*
chosen alternative solution is a managed runtime (QML)

Why not

it's always only asked for Plasma, but not for e.g. KRunner of KWin which also load plugins
it's only since Chromium started with it
WebKit took different approach to what Chromium does