[FEATURE REQUEST] Improve search in iOS13

[mneuwert]

Is your feature request related to a problem? Please describe.

• Adapt new UI improvements in UISearchController (ability to control when e.g. results are shown)
• Adopt UISearchTextFiled and UISearchToken APIs.

Describe the solution you'd like
I think we could improve search a lot by adopting UISearchToken similar to how Files and Photos app do it. Just try how it works in a Files app:

• As soon as search term matches some predefined token (e.g. document type), suggested tokens are appearing along with search matches. E.g. in a Files app, as soon as you type PDF, you'll see files matching it and as first item in the list you'll see "PDF Document".
• If you select this item, string "pdf" you just typed will be replaced with the token.
• Search text field can handle multiple tokens which precede the search text.
• Tokens can have an icon and you can copy them, drag and drop across apps or multiple windows.

We need 2 things technically:

• Way to match text user types to some collection of predefined tokens
• Transformation of list of tokens + text into a OCQueryFilter

Additional context
Screenshots from the Files App:

TASKS

• Research (if needed)
• Create branch feature/feature_name
• Development tasks
  • Implement whatever
  • ...
• Code review and apply changes requested
• Design test plan
• QA
• Merge branch feature/feature_name into master

PR

• App
• Library (if needed)