[applets/systemtray] Rewrite popups with layouts
Currently the code that draws plasmoid popups utilizes an anchor based approach.
I think layouts are a more elegant solution so this patch makes use of them.
Subscribers: safaalfulaij, manueljlin, ognarb, ngraham, kmaterka, mart, GB_2, plasma-devel
Differential Revision: https://phabricator.kde.org/D24720