Add a static QWindow::fromWinId(WId id) constructor which can be used to
create a QWindow object representing windows created by other processes.
Then, QWindow::setParent() can be used to embed a window into a foreign
window socket and QWindow::setTransientParent() to stick the current
window on top of a foreign window.

The changes in the QtWidgets module ensure that the focus chain (TAB
navigation) correctly works when a QtWidgets-based window is embedded
into another application.

As far as the platform implementation is concerned, this commit only
implements the embedding functionality in the XCB plugin. So, this is
roughly equivalent to the Qt4 QX11EmbedWidget functionality.

Change-Id: Iff8f7b9ee974d33fb30f36056f7838b433a413c7
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>