Broadcast application menu on PWM interface initiaization

Summary:
By the time AbstractClient::setupWindowManagementInterface() is called,
it's possible for a window to already have an application menu, and not update them
afterwards.

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D28150