Theme Tab - Reset Behavior - Introduce an option to reset all user-defined settings and display a theme in its initial configuration

[JaneSjs]

T16845 - When selecting another theme, the customized background and other properties remain between the themes https://surveyjs.answerdesk.io/internal/ticket/details/T16845

---

Usage scenario: When a user changes a theme, handle this and reset all user-defined settings and display a theme in its initial configuration.

So, we need the following APIs:

• An event which is raised when a user changes a theme;
• A function/method to programmatically reset a theme. The theme should appear in its initial configuration, e.g., without user-defined settings.

[tsv2013]

Customers can use the following code:

    const options = {
        showThemeTab: true
    };
    const creator = new SurveyCreator.SurveyCreator(options);
    const themeTabPlugin = creator.themeEditor;
    let isResettingChanges = false;
    themeTabPlugin.onThemeSelected.add((s, o) => {
      if(!s.isModified) return;
      Survey.settings.confirmActionAsync("Do you want to reset all changes?", (confirm) => {
                if (confirm) {
                    if(isResettingChanges) return;
                    isResettingChanges = true;
                    s.model.setTheme({ themeName: s.model.themeName, isPanelless: s.model.themeMode === "lightweight", colorPalette: s.model.themePalette });
                    isResettingChanges = false;
                }
            });
    });

Here is the live example - https://plnkr.co/edit/BMTNURUoIZhZyEwJ