Fix raptor widgets background priority vs jQuery-UI

Hello.

I'm using Raptor in no conflict mode. 
I have jQuery-UI already loaded on my site (but not on all pages).

When I open a widget (link, file...), images are not showing up. 
After a quick look on CSS, it's because jQuery-UI properties are taken instead of Raptor's. 

```
.ui-state-default .ui-icon {
    background-image: url(../img/build/jquery-ui/ui-icons_6da8d5_256x240.png);
}

.ui-icon, .ui-widget-content .ui-icon {
    background-image: url(../img/build/jquery-ui/ui-icons_469bdd_256x240.png);
}
```

have more priority than 
```
.ui-icon {
    width: 16px;
    height: 16px;
    display: block;
    background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAABZACAQAAAC9epbpAAC8f0lEQ…UsiPtUZwEM6QllOIlxZ7+J3sZLfvvKZAvg0xQCpH/Fmvr/AxpJB+o29SJYAAAAAElFTkSuQmCC);
```

So I update mammoth theme to use raptor css properties before jQuery-UI.