Minor changes to be properly compiled with `ng build --prod` command in Angular 8

[foncho]

We are developing a project with Angular 8, and we used this module as image carousel, but when we tried to compile it under production environment (ng build --prod), we have no able to finish the process due to some syntax errors like:

• onMouseOver() and onMouseLeave() methods become to public, instead of private.
• prevImage() and nextImage() called from image-viewer.component.html file, become to prevImage($event) and nextImage($event), because of building process were failing if you did not pass any argument to these methods, which are trying to handle one (event) in the code.

Hope you may wish to consider these minimal changes for improving this awesome module.

[chrismclarke]

Would appreciate this one getting merged in as current production build breaks when using with angular 8