possibility to use a fallback chain as prefix

Summary:
if a framesvgitem has an array as prefix, like

FrameSvg {
prefix: ["toolbutton-hover", "hover"]
}

it will use the first available prefix, so on themes that
have toolbutton-hover, that one will be used, old themes will
continue to use "hover" as prefix

Test Plan:
tested the calendar with breeze theme has breeze-widget style
toolbuttons, with olt themes as air, the old behavior is still
there, switching on the fly works

Reviewers: davidedmundson, Plasma

Subscribers: davidedmundson, broulik, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4827