Layout Directory: Add Category & Niches Support

[AlexGStapleton]

Resolves https://github.com/siteorigin/siteorigin-panels/issues/428 Requires https://github.com/siteorigin/layout-viewer/pull/8

• Added category support. Only one category is selectable at a time.
• Added niches support. These act as tags and multiple can be selected at once.
• Added filtering without additional server requests.
• Added caching (also added in dedicated PR). Currently disabled to help with testing.
• Started groundwork for Favourites.

A build is required to test this PR. To test this PR:

• Set up this PR on a dedicated local site.
• Set up a bunch of test layouts (be sure to assign a category and niche for each one).
• Adjust this line to reference your test site.

[AlexGStapleton]

Search has now been added to the Layout Directory. I'm not seeing any issues with the other views so I'm not marking this as ready for review.