Adding search with highlight all

Hi there, I'm currently trying to work on adding this long time feature request: https://github.com/mlouielu/vte/tree/search-highlight-all

My init thought is, maintain a selection_resolver array, when performing search_next, search_previous, it will update the match into the array, which in the screen buffer visible parts.

For the API, I'm not sure if we need a separate one or not (e.g. search_highligh_all())

Any thought on this? thanks.