OS X: Improve QWindow embedding support

Improve support for embedding QWindow in 'foreign' view hierarchies.

Use cases under consideration:

• Qt as a plugin: Qt is used to author a plugin for a native application, where the plugin typically provides a native view instance which the host application controls.
• NSWindow configuration: There are many ways to configure a NSWindow and we can't support them all through the cross-platform Qt API. Instead the application author can configure the NSWindow they like and then embed the Qt view in it.

Other work

"Add foreign window internal documentation": https://codereview.qt-project.org/#/c/127152 (https://gist.github.com/torarnv/c5dfe2d2bc0c089910ce)

Current support

QWindow::winId()
QMacNativeWidget

Plans

Make "Qt is embedded" the default case on OS X: Don't make any assumptions on what the parent view of the Qt view is. The normal configuration where Qt is control of the entire hierarchy is then a special case of this, and we seek to minimize any special handling required.

QMacNativeWidget will be deprecated. You don't need special widget, instead you embed the top-level Widget (or more specifically the window for the top-level widget)

Suggested Changes

Native View Accessor API
A safer, more convenient replacement for winId() and windowHandle().

NSView *QWindow::nativeView()
NSView *QWidget::nativeView()

• Returns a platform-native view type (NSView on OS X, etc)
• Creates the backing QWindow/QPlatformWindow as needed
• Configures the platform window for “embedded mode” (if needed)
• Transfers ownership of the QWindow to the native view. (On platforms were the native type can own things)

Native View Accessor API V2

NSView *QWindow::toNSView()
static QWindow::fromNSView()

https://codereview.qt-project.org/#/c/145488/

QWidget Visibility
We would like to control visibility from the from the platform plugin in cases where the widget is shown using native API.

QWidget *root = new ApplicationWidget();
NSView *view = root->nativeView();
[[window contentView] addSubview:view];

// later:
[window makeKeyAndOrderFront];

Problem: QWidget needs a setVisible(true) call to transition to the correct state where it will paint.
Solutions:

• Make QWidgetWindow::handleExposeEvent() force visibility.
• Add QWindowSystemInterface::VisibilityEvent