ConfirmationDialog is missing horizontal button separation style

### Tested versions

v4.2.2.stable.official [15073afe3]

### System information

Ubuntu

### Issue description

The ConfirmationDialog introduces a second button to the AcceptDialog, showing the two options side by side.

This should also come with a themable property to specify the ConfirmationDialog's internal HBoxContainer's `separation` override.

### Steps to reproduce

1. Try adding `ConfirmationDialog` styling to your theme.
2. Note that there is no styling available for `ConfirmationDialog`
3. Note that there is no way to easily modify the gap between the cancel and the accept button in a `ConfirmationDialog` (it would probably be possible via code, but thankfully we have a theming system for such things.)

### Minimal reproduction project (MRP)

n/a

Start by locating ConfirmationDialog, its AcceptDialog base, and the internal HBoxContainer that holds the two buttons. Review how existing themable container separation properties are exposed, then verify that a theme can control the gap between the cancel and accept buttons.

Written by the indexing model from the issue text.