[UX] Layouts: Visibility conditions (and contexts): Entity ID text fields with autocomplete fields (or entity browser?).

Follow-up to #1077...

It would be great if the respective fields for node_nid, user_uid, file_fid and taxonomy_term_tid would be able to do auto-complete search and offer pretty entity names to pick from instead of the numeric fields we are currently providing:

(form elements in the screenshot above edited for visibility)

We should also replace those "cryptic" names in the drop-down menu with pretty names too like "Content: Title", "User: Username" and "Taxonomy term: Name".

The value saved in config would still be the entity ID, we'd only show a "pretty", human-focused autocomplete to the end user, with available options to select from.

PS: while at it, also replace "Node: Type" with "Content: Type" in the spirit of not exposing the term "node" in the UI.