Bug 903002 - Display properly the widget with panel attached, in Australis

WIP, unit test is still missing.

The code had to be modified again because a weird behavior I noticed when the widget is added the first time, see [my comment](https://bugzilla.mozilla.org/show_bug.cgi?id=880458#c7) in [bug 880458](https://bugzilla.mozilla.org/show_bug.cgi?id=880458); so basically checking for the new attribute `customizableui-anchorid` is not enough, we also have to check if it's the widget is actually in a panel or not.