Popup menu search

[philippfromme]

• adds ability for popup menu providers to provide custom search
• default search sorts matches by location (when searching for "foo", "foo bar" will appear before "bar foo")

[philippfromme]

Example usage here: https://github.com/camunda/camunda-bpmn-js/pull/367/files#diff-1abbb6d8413b72aac12009eae34de187cdc46f9c9eeb5daad527e5c7c43a8df8R23

Not entirely shure if this is the way to do it or wether we should have separate providers for search similar to: https://github.com/bpmn-io/diagram-js/blob/develop/lib/features/search-pad/SearchPadProvider.ts