Better handing of file updates during search.

ThunarListModel now knows if it's representing a search result, and what the active search terms are. This allows it to filter new files when needed, according to the active search query. No more non-matched files showing up in a search results listing if they're created during a search and are spotted by folder monitoring.

Fix #787 (closed). I hope this isn't breaking the optimization for regular single-folder contents models that uses the ->hidden list. I've left the assertion in there for the non-search case.

Address #801 (closed) slightly by making deeper monitoring more possible.