Adopt EventEmitter class instead of injecting Signal methods

Introduce a new class, EventEmitter, which implements signal
handling for pure JavaScript classes. EventEmitter still
utilizes GJS' addSignalMethods internally.

EventEmitter allows static typechecking to understand the
structure of event-emitting JS classes and makes creating
child classes simpler.

The name 'EventEmitter' mirrors a common name for this pattern
in Node and in JS libraries.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2043>

js/js-resources.gresource.xml

@@ -29,6 +29,7 @@
     <file>misc/permissionStore.js</file>
     <file>misc/signalTracker.js</file>
     <file>misc/smartcardManager.js</file>
+    <file>misc/signals.js</file>
     <file>misc/systemActions.js</file>
     <file>misc/util.js</file>
     <file>misc/weather.js</file>