Fix ComboButton click on webkit mobile (iOS and android).
Set dijitOffScreen class on <input> so dojo/touch doesn't squelch the
click event generated from _ButtonMixin's call to this.valueNode.click().

Fixes #17693.
fix test typo and update source code comment, refs #17693
Widgets that use ondijitclick in their templates should list dijit/a11yclick
as a dependency.  Refs #17693 tangentially.
fix over-eager setting of aria-invalid="true", fixes #17928
Implement ConfirmDialog and ConfirmTooltipDialog.

Related changes:
- Make _getFocusItems() scan this.domNode rather than just this.containerNode,
  in order to find the OK/cancel buttons too.
- Change Dialog's close icon from tabIndex=0 to tabIndex=-1, so it's not in the tab
  sequence.
- _getFocusItems() doesn't take a parameter, so stop passing one.

Fixes #16930, fixes #13279.