other preload microoptimizations

Summary:
make svgitem schedule a polish right when it's done loading

ScrollView now forces to layout items as soon is created, to preload items
before the window is shown

remove useless include

Test Plan:
possible to preload (almost) the whole systray with less hacks
on that part (see D10692)

Reviewers: Plasma, davidedmundson, broulik

Reviewed By: Plasma, broulik

Subscribers: davidedmundson, broulik, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D10722