Get rid of major copy-paste of core sharedialogview's autocompleteHandler

[PVince81]

We need to extend the function, not copy-paste it:

var oldHandler = obj.autocompleteHandler;
obj.autocompleteHandler = function(search, response) {
    return oldHandler.call(obj, search, function() {
        // add additional entries here
        // ...
        response();
    });
}

One challenge though is that the result is already rendered, including the "No users" entry if there was no user. So one would need to override this as well.

Best would be to have another hook / callback in core that processes the results and that can be overridden. Maybe moving the autocompleteHandler's result handling code into the dialog as two methods _queryAutocompleteResults and _processAutocompleteResults, which would allow apps to hook in.

[PVince81]

@pmaier1 tech debt here. If we ever make adjustments to core's autocomplete dropdown code, we'd need to copy it into this app as well as it's been copy-pasted. I can help clean this up but might need some core modifications to allow easier extensibility of the autocomplete handler.

[PVince81]

... and failing to copy said changes would have instances display the "old" behavior whenever the guest app is enabled, as it overrides the whole share autocomplete results handling.

[PVince81]

improved in #149