Make all io process non-blocking and cancellable

### Problem

There are extant reports of Files freezing/hanging under some circumstances and this is most likely due to using blocking calls like `query_exists ()` and `query_filetype ()`. e.g. #1858

Another example of this is loading a large folder (especially into IconView) - the rest of the interface freezes while the view is loaded and drawn. This may be partially alleviated by using dynamic views in Gtk4 though.

### Proposal

We should always use asynchronous/cancellable calls to stop the whole interface freezing.

### Prior Art (Optional)

_No response_