[applets/systemtray] Rewrite popups with layouts

Summary:
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.

Test Plan:

Reviewers: Plasma, VDG, ngraham, mart

Reviewed By: Plasma, VDG, ngraham, mart

Subscribers: safaalfulaij, manueljlin, ognarb, ngraham, kmaterka, mart, GB_2, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24720