[KDeclarative] Port away from deprecated QQuickWindow::sendEvent

Summary:
The code in QQuickWindow::sendEvent doesn't do anything special
for UngrabMouse events, it just calls QCoreApplication::sendEvent
[This code says QGuiApplication::sendEvent everywhere so I used
that for consistency]

Test Plan: Builds

Reviewers: mart, davidedmundson, aacid

Reviewed By: aacid

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T11553

Differential Revision: https://phabricator.kde.org/D23896