support for icons in buttons

The Button and ToolButton components in QtQuickControls2 do not support icons in the buttons anymore, this is a regression compared to both QWidgets and QtQuickControls1, especially for ToolButtons that are usually either icon+text or icon only, making the control not very useful, especially since i'm trying to realize a desktop friendly QtQuickControls2 style.

Even just a string property in the template class would be nice, and would be up to the style to either implement the icon or ignore it completely if some platform never wants icons on buttons