Itemfilter refactoring/rewrite

[silvester-pari]

Current eox-itemfilter functionalities:

• aggregate filter choices based on inputted items (frontend search) - for backend search define choices
• "widget" mode (filters are stacked in a column, results underneath) and "inline" mode (filters all live inside one "input" form element, like e.g. GitLab UI)
• frontend search (powered by fuse.js or similar) and backend search (filter inputs can be injected into a endpoint string template and are sent to an API and results shown)
• reset individual filters, or reset all filters
• grouping of results by key
• custom styling of filters via e.g. slots or CSS pseudo-elements (before/after) with data-attrs
• start itemfilter with pre-set filter state

Filter types:

• text search (ideally with fuzzy search)
• single select
• multi select
• range
• spatial

Currently, our select filters use Fuse.js behind the scenes to do exact matches (e.g. =text).

Potential evolution:

• accessing nested properties (.) see Nested property filtering #603
• provide multiple result rendering approaches, as web components (e.g. list, tiles, "statistical" table) potential future evolution