Add controls to the "related category" widget so that it is possible to specify which categories should be included in the grid

The categories grid Acorn currently populates with all of the categories of the website. While often the desired behavior, there are times when it's desirable show a portion of the categories in the site. For example, perhaps a website has a "technical" and "business" blog. While there might be some pages which overlap in audience, it might be desirable to exclude the technical categories form the business blog or vice-versa.

Current articles grid (taken from www.oak-tree.tech/blog):

Desired features:

• Create a "categories" selector widget similar to those in wgtcontent for vectors or HTML snippets
• Create a "category" block that can be included within a "category grid"
• If a list of categories is present, render only those categories. If no list is present, render all categories for the Acorn instance.