Moved mask-related styles out of Panel and into WidgetModality.

Panel's skins now use `.yui3-panel` instead of `.yui3-panel-content`. Fixes 2531623

Improved support for Panel's [x] button. This sort of feels like too many hacks to get the concept of a single "close" button that renders as a normal button [Close] when in the footer, and as an icon button [x] when in the header. Feel free to convince me of something better :-/

Convert panel sprites to PNG. Files aren't actually smaller than the GIF versions, but they likely will be if more icons are added down the road. Plus, GIF is so 90s.

Moved predefined `buttons` and styles from WidgetButtons to Panel. Panel now defines the default "close" button, along with its styles for both skins.

Removed all .yui-buttons CSS styles from Panel's CSS.

Improved Panel's CSS and button layout. WidgetButtons' `buttons` attribute now support a `classNames` property which will add CSS classes to the button Node. The default close button uses this to add `yui3-button-close` CSS class to itself. This change made it much easier to target the default styling of the close button and the styling that the Panel skins add. Relates to #2530952 Panel got its CSS reworked a bunch, especially for the CSS responsible for laying out the Panel's buttons. The `panel-core.css` is now much cleaner and only has style rules for the things that are layout specific while being skin agnostic. The styling for Panel's header is now much more flexible, while maintaining the position of the close button across browsers and various amounts of content. Fixes #2530978 Panel's Sam Skin CSS got a once over to tidy it up, while Panel's Night Skin underwent some major updates to make it's style work across more browsers.

Removed `.yui3-panel` selector from CSS rule which sets `float: right;`. Added newline to the end of sam panel-skin.css.

Removed `.yui3-panel` selector from CSS rule which sets `float: right;`. Added newline to the end of sam panel-skin.css.

panel css changes to improve the positioning of the close button

fix test issue in IE + css fixes to panel to make it look decent in IE

update css + add sprite to panel as made my jeff. also update the manual test to reflect api changes made to widget-buttons

update panel css

skin Y.Panel + use newly created extensions

initial commit for panel