[PlasmaCore.IconItem] Refactor source handling for different types

Summary: There are 3 possible strategies: QIcon, QImage and SVG. This change moves logic of each of these strategies into separate class.

Test Plan: Should behave exactly the same

Reviewers: Plasma, broulik, apol, davidedmundson, Frameworks

Reviewed By: Plasma, davidedmundson

Subscribers: mart, davidre, cblack, kde-frameworks-devel, Plasma

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28470