Add support for specifying max width

[FaizanDurrani]

Checklist

• [x] I've tested my changes.
• [x] I've read the Contribution Guidelines.
• [x] I've updated the documentation if necessary.

Motivation and Context

Wanted more control over the width of the menu; Also fixes #33

Description

Added a property in PopMenuAppearance that allows you to set the minimum width the menu should try to maintain. Setting it to nil or .none will revert to the old implementation.