ConfirmationDialog is missing horizontal button separation style

[h0lley]

Tested versions

v4.2.2.stable.official [15073af]

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