Ability to define a custom stylename for a dialog action

[glebfox]

See support forum.

---

changes

DialogAction class has withStyleName() method and can be configured in the fluent style:

new DialogAction("ok")
        .withStyleName("ok-style")
        .withCaption("OK")
        .withHandler(event -> {
              // do smth
        });

QA

1. Create option dialog with custom DialogAction. Add style name to this action:

dialogs.createOptionDialog()
        .withActions(new DialogAction("ok")
                .withStyleName("ok-style")
                .withCaption("OK")
                .withHandler(event -> {
                    // do smth
                }));

2. Extend theme and add the style that is used in the above code:

.ok-style {
  background: greenyellow;
}

2. Launch the app and open dialog.
3. Button should be colored.