Dialog: Remove stack option

[Scott González]

The stack option should just be removed and dialogs should always be at the top when focused.

This is not being deprecated; this is being removed immediately.

[maciejmrozinski]

​https://github.com/jquery/jquery-ui/pull/791

Should we leave force option for moveToTop function? In API docs this function has no arguments.

[Nate Eagle]

Dialog: Awesome new stacking and overlay implementation. Fixes the following tickets:

Fixes #3534 - Dialog: Modal dialog disables all input elements on page. Fixes #4671 - Dialog: Modal Dialog disables vertical scroll bar in Chrome & Safari. Fixes #4995 - Dialog: Modal Dialog's overlay dissapears in IE when content is tall. Fixes #5388 - Dialog: Don't change z-index when already at the top. Fixes #5466 - Dialog: "modal" Dialog Incorrectly Cancels Input Events. Fixes #5762 - Dialog: Get rid of z-index workaround, document it instead. Fixes #6267 - Dialog: checkboxes that inherit a z-index < jqueryui.dialog z-index don't work. Fixes #7051 - Dialog: modal prevents tab key from moving focus off slider handle. Fixes #7107 - Dialog: Modal dialog event loss with high zindex child elements (FF 3.6). Fixes #7120 - Dialog: Modal operation interrupts drag drop marker functionality on gmaps. Fixes #8172 - Dialog: Change event cancelled when opening modal dialog from another modal dialog. Fixes #8583 - Dialog: Mouse event wrongly stopped. Fixes #8722 - Dialog: Remove stack option. Fixes #8729 - Dialog: Remove zIndex option.

Changeset: 3829a37ca122e923c3a08b964c4b1a946a2a1456