Remove code duplication between Confirm[Tooltip]Dialog and [Tooltip]Dialog

This removes the duplication  between the four dialog widget templates by introducing an `${!actionBarTemplate}` variable into the (original) `Dialog` and `TooltipDialog` templates.

It also adds a `_ConfirmDialogMixin` class shared by `ConfirmDialog` and `ConfirmTooltipDialog`.

@csnover, what do you think?  

I thought about other ways to do it too, like giving `_DialogMixin` a `buttons[]` property to list all the buttons to put in the action bar (with the default being an empty array), but not sure that's better, or worth the effort.   The approach in this PR is probably more customizable.

Refs #16930, refs #13279.

PS: The kind of customization I'm worried about is if someone wanted to stick a ComboButton (or DropDownButton) in their action bar.