Consistent QWindow close event behavior

Closing a top-level QWindow should have consistent behavior regardless of API used or QWindow subclass in use.

Desired behavior includes:

• deleting the platform window
• application last-window-closed logic

Close APIs include:

• QWindow::close()
• QWindowSystemInterface::handleCloseEvent()
• QWidget::close() (QWidgetWindow)
• QtQuick.window.close() (QQuickWindow)

Requirements:

• Run all close logic at the various levels
• Run close logic in the correct order.
• Deduplicate logic as far as possible.

Previous Work

Some work has been done already for 5.6 and QTBUG-43344. This was partially reverted since it caused the regression described in QTBUG-50854.

Implementation

Control Flow
1) All close API, for top-level QWindows, redirect control to QWindow::close() or QWindowSystemInterface::handleCloseEvent(). QWindow::close() calls handleCloseEvent().
2) The QWindow subclass handles the CloseEvent. It runs subclass-specific close logic, and then calls QWindow::event() to pass the event to the QWindow base implementation.
3) QWindow runs common close logic:

• delete the platform window
• application last-window-closed behavior (but this is currently duplicated in QWidgetWindow)

TODO: who hide()s the window?

quit-on-close

Both QWidget and QWindow have quit-on-close logic. The one difference is that the QWidget version checks WA_QuitOnClose (widgets without that flag won't keep the app alive) while the QWindow version checks QGuiApplication::quitOnLastWindowClosed().

Keeping (and running) both code paths make sense, to catch usages of both WA_QuitOnClose and QGuiApplication::quitOnLastWindowClosed(). This is not the current behavior: instead, QWindow will only run quit-on-close logic for windows that it itself hides (and not for windows that QWidget hides).

Pending Changes

https://codereview.qt-project.org/158701 (Unify window close behavior)