Plugins: Update to new composer lifecycle

Update to be compatible with Application.Controller composer lifecycle
changes, add matching signals so plugins can decorate composers as
they are created.

Keep track of Plugin.Composer implementation instances so that the same
object can always be returned for the same Composer.Widget instance.

Update plugin call sites.

src/client/application/application-controller.vala

@@ -339,7 +339,7 @@ internal class Application.Controller :
             composer = new Composer.Widget(
                 this,
                 this.application.config,
-                context,
+                send_context,
                 null
             );
             register_composer(composer);
@@ -418,7 +418,7 @@ internal class Application.Controller :
             composer = new Composer.Widget(
                 this,
                 this.application.config,
-                account,
+                send_context,
                 null
             );
             register_composer(composer);