Add ability to clear items in Slideshow.Builder

[RinLovesYou]

Issue

Issue Checklist

Please follow the following steps before opening this issue.

Issues that do not complete the checklist may be closed without any help.

• I have checked for similar issues on the issue tracker.
• I have updated to the latest version of JDA-Utilities.
• I have checked the branches or the maintainers' PRs for upcoming features/bug fixes.

The issue tracker is reserved for issues, errors, and feature requests related
to JDA-Utilities, and not questions and other requests for help.

• For examples, check out the examples module or the list
  of JDA-Utilities projects.
• For questions join the official JDA discord server and ask them
  in our #jda-utilities channel.
• For general programming questions, visit StackOverflow.

Issue Information

Check all that apply:

• This is a bug report about an error, issue, or bug in JDA-Utilities.
  • I have been able to consistently reproduce this bug.
• This is a feature request for the JDA-Utilities library.

This issue tracker does not assist or handle issues with the JDA library.

For JDA related issues, visit the JDA issue tracker
and open an issue there.

Description

Describe your issue here.

Please provide any stack-traces, code, or pictures that will help describe the issue you are encountering.

The Paginator.Builder Class has the option to clear existing items with the function clearItems().
The Slideshow.Builder Class, while almost identical, is missing this function.
This causes previous images to still be in the pages, even after opening a new instance of the Slideshow.