Add a search filter to restrict items in the dropdown

Problem/Motivation

Option lists in multiselect dropdowns can potentially be quite long. It would be nice to have an option to limit the list with a search.

Proposed resolution

Add an optional search input to multiselect dropdowns which only shows matching results.

User interface changes

None, unless the option is enabled by the user.

API changes

A way to specify a custom filtering callback will be added.

Data model changes

New config keys added for:

• search_title
• search_title_display
• search_placeholder
• search_character_threshold