ksvg2icns: use QCoreApplication instead of QGuiApplication

ksvg2icns doesn't appear to need QGuiApplication; using QCoreApplication
instead makes it possible to use the tool on headless (CI) servers.
Alternatively it could use QGuiApplication with the "minimal" QPA.

it possible to use the tool on headless servers where the cocoa
QPA plugin is likely to fail.