This patch ports the close button from a PlasmaComponents.Button to an IconItem so as to:
- achieve better visuals -> the close button frame is ugly and unnecessary with Breeze
- achieve better symmetry -> the frame makes the close button appear indented
- achieve consistency with ToolTipInstance.qml -> also uses an IconItem for its close button
- address symmetry issues caused by the port -> headings are moved slightly upwards
- port the Configure button as well?
- add a fallback hover-aware circle for 3rd party themes whose "window-close" icons have no built-in hover effects?