shell: added patternfly styled dropdown menus

Fixes: #687
I see.  I think the canonical initialization is just ```$('.selectpicker').selectpicker()```, without any options.  (But please check this.)

In any case, the placing of this snippet is odd.  I would expect it to be run only once after shell.html is loaded.  I propose in ```init``` in cockpit-main.js.

If we dynamically create selectpicker elements, we should initialize them on the spot, like this:

```
var elt = $('<select>').selectpicker({ ... }).append(...);
```

In this case, it also makes sense to pass options.