The Title.changed signal could emit some detail about the change

[krassowski]

Problem

In jupyterlab/jupyterlab#13676 (comment) we would like to filter out updates caused by tab switch, which changes className of tab title leading to the signal being emitted:

lumino/packages/widgets/src/title.ts

Lines 225 to 231 in 06a87ea

	set className(value: string) {
	if (this._className === value) {
	return;
	}
	this._className = value;
	this._changed.emit(undefined);
	}

However, sine it is emitted with undefined, we cannot distinguish the cause of the change and filter out specific updates.

Proposed Solution

Change the signal to include the update information.