Decouple active window from focus window

Window focus is not the same as window activation.

A single window (possibly non-Qt) window is the focus window (receives keyboard input e.g., but not limited to), and is typically also active, but other window may also be active:

• siblings of that window
• children of that window
• parents of that window
  • including transient parents
• other windows, that the OS considers active even if another window has keyboard focus

Some things we need to clean up:

• Widgets
  • Replace tests calling QApplicationPrivate::setActiveWindow with requestActivate
  • Replace QApplicationPrivate::active_window with QGuiApp::focus_window
  • Simplify QApplicationPrivate::setActiveWindow if based on focus window
  • Replace calls to QApplicationPrivate::setActiveWindow with update of focus window and let code in QApplicationPrivate::setActiveWindow trigger as a result, not a cause
• Gui
  • Plumb QWindow::isActive() to QPlatformWindow::isActive()
    • Remove QWidget::isActiveWindow() plumbing for the same
  • Rename QWSI::processActivatedEvent to focusWindowChange
    • https://codereview.qt-project.org/c/qt/qtbase/+/519851 
  • Add dedicated QWSI for window activation change only
  • Emit activeChanged along with notifyActiveWindowChange
  • Deprecate Qt::WindowActive in favor of QWindow::isActive()
  • Send/emit activation change before focus changes in QGuiApplicationPrivate::processFocusWindowEvent
    • Logically makes sense, the window is activated first, and as a result, gains focus
    • Matches the order that widgets receive the events in QApplicationPrivate::setActiveWindow
• Testlib
  • Add qWaitForWindowFocus(), to distinguish it from qWaitForWindowActive