Add EmbedPaginator to menu module

[Andre601]

Pull Request

Pull Request Checklist

Please follow the following steps before opening this PR.

PRs that do not complete the checklist will be subject to denial for
missing information.

• I have checked the pull request page for upcoming
  or merged features/bug fixes.
• I have read JDA's contributing guidelines.

Pull Request Information

Check and fill in the blanks for all that apply:

• My PR fixes a bug, error, or other issue with the library's codebase.
• My PR is for the menu module of the JDA-Utilities library.
• My PR creates a new module for the JDA-Utilities library: ______.

Description

Closes #103

This implements a EmbedPaginator class to the menu module, which allows to pass own instances of a MessageEmbed to make paginated Embeds.

This is mostly based on the Slideshow class.

There are methods that where not implemented for multiple reasons:

• setColor and showPageNumber: Seems weird and would probably be a quite "hacky" aproach to alter an Embed's colour if you provide it through the MessageEmbed instance itself.
• setItemsPerPage: You can only have one embed per page, for obvious reasons.

I also added a addItems(String... items) and setItems(String... items) method to just create a basic Embed with nothing more than the description. That is just a convenience method in case someone wants this.

[Andre601]

I pushed some minor corrections to the overall format.
In addition did I also add a Checks.check(String#length() <= MessageEmbed.TEXT_MAX_LENGTH, "Message"); check for the addItems(String...) and setItems(String...) methods to make sure that none of the provided Strings exceeds the embed limit.

[Andre601]

Added a addItems(Collection<MessageEmbed>) and setItems(Collection<MessageEmbed>) option.