Replace current icons in the core by using the new Icon-API
The logic for working with icons, icon sizes and icon overlays is now bundled into the new ``IconFactory`` class.
The new icon factory will replace the old icon skinning API step by step.
All core icons will be registered directly in the IconRegistry class, third party extensions must use
IconRegistry::registerIcon() to overwrite existing icons or add additional icons to the icon factory.
The IconFactory takes care of the correct icon and overlay size and the markup.
The replacement should be done for each icon. So that is easier to handle and better for reviewing.
Each icon gets a sub-issue that is related to this story.