A11y issues : Thumbnail alt and Previous/Next buttons role

[trystan2k]

Hi !

What is the expected behavior?

We are using axe-core (https://github.com/dequelabs/axe-core) to test the accessibility of our website (that uses ngx-gallery) and we found 2 issues related to missing attributes.
We expect to not have any a11y errors when using ngx-gallery

What is the current behavior?

The current errors are: in the thumbnails images, that miss the alt attribute (which is already set in the main image) and the previous and next button give us an error saying that the i element cannot have aria-label attribute without a role attribute.

What are the steps to reproduce?

Just create a new Angular project with ngx-gallery (the StackBlitz starter, for example) and add a unit test with axe-core

  .run()
  .then(results => {
    if (results.violations.length) {
      throw new Error('Accessibility issues found');
    }
  })
  .catch(err => {
    console.error('Something bad happened:', err.message);
  });```

#### What is the use-case or motivation for changing an existing behavior?
Making ngx-gallery more accessible to everyone.


#### Which versions are you using for the following packages?

Angular: 14.2.2
Angular CDK: 14.2.2
Angular CLI: 14.2.2
Typescript: 4.8.3
Gallery: 7.0.4


#### Is there anything else we should know?
I can provide a PR later with these fixes, if it is ok for you.