Add foundation for displaying data of entities themselves

[versusbassz]

The quite large task. Added a project for that: https://github.com/versusbassz/wp-meta-viewer/projects/2

Sub-tasks

• [x] refactor backend of meta fields to a Fetcher-class
• [x] "mysql" order for entity props
  • [x] Fix visual problems (the width and wrapping of the "DB order" column in different situations)
• [x] memorize a chosen tab (by entity type, stored in localStorage as a json hashmap)
  • [x] TTL for stored value --- See: #62
• [x] Improve displaying of tabs/sections names (human readable names, not ids)
• [x] to do smth with post_content (related to #2)
• [x] Handle wp_errors on the backend properly + display them on frontend

Misc

• [x] add an issue for better displaying of terms (maybe, several tables in one entity tab) --- See: #51
• [x] custom order (more convenient) for props of posts (to a separate issue) --- See: #52

[versusbassz]

Most of the task has been done. Several sub-tasks left (polishing).

[versusbassz]

Done. This is a big step forward :cake: