Add Icon component to Controls

Qt Quick Controls has some components (e.g. AbstractButton and all derived components) that show icons. These icons are set using properties such as icon.source. When displayed, they will change color depending on the current color scheme (e.g. Material.Dark will display all icons as white). Sometimes, such icons would be useful to use without being attached to an AbstractButton, MenuItem, or other such component; instead, they could be shown as context for a Label. I propose that the existing icon code be reused to create an Icon component that has approximately the following API:

// Icon.qml
Control {
    property string source // path to icon file
    property string name // used to choose a system icon on Linux
    property color color // used to override the icon's color
    property bool cache: true // caches the icon
}

This API is largely copied from existing icon interfaces.