Icon: Fix tinting of icons to work as described by AbstractButton.icon

The QQC2 AbstractButton component's icon property states that the icon is tinted unless the color is set to "transparent" so Kirigami should exhibit the same behavior. This removes the heuristic based on icon names ending with the "-symbolic" suffix, and adjusts logic to match Qt documentation for the icon property.

In particular, this fixes an issue where Kirigami does not apply the tint to icons loaded from qrc:// or directly in the filesystem, when using the qqc2-breeze-style with QML applications.