Popup: blue focus flash on android 4.2

So I tested this with Browserstack on android 4.2 and it fixed the problem.  However, when I run "grunt test" there are a number of test cases failing.  @arschmitz suggested I do a pull request so he can take a look.  So don't merge if it's breaking tests! :)
As we discussed with @arschmitz, we should improve the focus handling so the chain is not initially broken. The way to do that is to find an element inside the popup container that can be focused, and use that for ```this._ui.focusElement``` instead of using the container itself. Of course, if we do not find such an element, then it is right that the focus chain be broken, because *nothing* can be focused while the popup is open.

So, basically:

1. Find a focusable element within the container. For this we can use the ":focusable" selector provided by jQuery UI.
2. If we find such an element, we assign it to ```this._ui.focusElement```, which ```_handleDocumentFocusIn()``` later uses to redirect focus away from an element that is not part of the popup and onto an element that is.
3. Profit!