Bug 1098168 - BaseIcon

It does not seem right to me statusbar.js need to check if mozNfc then load nfcIcon; if mobileConnection then load mobileConnectionIcon.

The main problem is that we need ? to put the icon in certain ordering. And DOM element could not be re-ordered. If we don't need to care about icon ordering, the semantic will be better - module loads and renders its own icon. What's your best concern?