SvgItem, IconItem: drop "smooth" property override, update node on change

Summary:
QQuickItem already has a "smooth" property, which was shadowed by the
custom ones with the same name. As the property was not part of public
symbols, but instead is dynamically looked up without the class name part
of the property, we just remove the custom versions and instead switch
all custom code to use the inherited property.

While doing that the code is also fixed to properly update the
textureNodes to the current value of the "smooth" property.
As well as not set the filtering only on the texture, when it is passed
to a texturenode which will set its own filtering state to the texture
right before the bind call and thus wipe any previous direct setting
of the filter mode on the texture.

This also changed the default to smooth=true for SvgItem, though
effectively smooth was always true, as the texturenode was hardcoded
to get a QSGTexture::Linear filtering (which, as said above, is forced
onto its texture, no matter what was otherwise set before).

Reviewers: Plasma, davidedmundson, mart

Reviewed By: Plasma, davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20510