Added "icons" option for UI Dialog buttons

[alexeevdv]

It allows you to add custom icons for dialog buttons.
You can see example here: http://jsfiddle.net/alexeevdv/g5pYa/

[jzaefferer]

This overlaps with #423, which also forwards a "showText" property. I think what we really need is a way to customize all button options, especially since button is becoming a required dependency for dialog (when using the buttons option).

[alexeevdv]

i added a list of allowed button options which will be trimmed from common options. if text is not specified or false it will not shown. You can see it on example page: http://jsfiddle.net/alexeevdv/g5pYa/

[scottgonzalez]

The problem with having a pass-thru to the buttons API is that we currently have a pass-thru to .attr() and they don't mix well. There was already a discussion about this in http://bugs.jqueryui.com/ticket/6830 and the best API seems to be showText and icons in addition to full support of .attr().

[jzaefferer]

Buttons should always have text, so closing this in favour of #423. Which I'm probably not going to merge either, as it doesn't have tests and needs to be rebased on the dialog branch.