Add "primary" property for StandardButton (Dialog context)

[antis81]

Feature Description

A small followup issue to #7240 just to get it out of my head (did not find a similar issue addressing this).

For Simple yes/no dialogs setting the input focus on the button is just fine. However when the use case gets slightly more complex and an additional input field also wants focus (for example a password dialog) this approach no longer works. In this example setting the input focus to the password field "steals" focus from the "ok" button.

The slint Button reference already discusses the primary property. Does this apply to StandardButton as well?

Product Impact

Yet another "quality of life" feature.