FilesEngine is a redundant interface with HasFileData and access to inner event types

[jkelleyrtp]

Specific Demand

We expose the FileEngine object which used to basically be a cache of the renderer's native event, allowing access to dragged/dropped files. Now that events have the native event type in them, this is no longer necessary! We should simply move these methods on the events themselves, simplifying the API and simplifying code that uses them.

For example, the snippet from our file_upload example

        if let Some(file_engine) = &evt.files() {
            let files = file_engine.files();
            for file_name in &files {
                if let Some(file) = file_engine.read_file_to_string(file_name).await {
                    files_uploaded.write().push(file);
                }
            }
        }

could just be simplified to

            for file in evt.files() {
                if let Ok(contents) = file.read_to_string().await{
                    files_uploaded.write().push(contents);
                }
            }

This might require a type like a VirtualFile or something like that that implements the Read trait - or some reworking of methods, but overall the FileEngine object is awkward now.