Add selected jQuery-UI layout helper-classes

[StevenBlack]

I suggest adding the following jQuery-UI helper classes to jquery.mobile.core.css.

https://github.com/jquery/jquery-ui/blob/master/themes/base/jquery.ui.core.css#L13-20

These layout helpers for hiding, resetting, and clearfixing are very handy, and they nudge the framework users towards doing these common tasks in a standard way.

[toddparker]

Assigned to Scott to review what he thinks we should port over from UI to mobile.

[toddparker]

Added to the feature request wiki page:
https://github.com/jquery/jquery-mobile/wiki/Feature-Requests