Cocoa: Bring back two-class native window system

While inheriting from NSPanel proved to be robust enough, it is
not really future proof as we're at the mercy of Apple changing
NSPanel's behavior. On the other hand, we can't inherit exclu-
sively from NSWindow as the tool window case, where the QWindow
should look like an NSPanel, can't be emulated perfectly without
using private APIs.

This reverts commits 79fb39a87ce5ffbda and df86721bb4028ae.

Change-Id: I9021193e3614633a943578df9e2794b00094a1f7
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Check whether exposed widget is mapped in QWidgetBackingStore::sync(QWidget*).

Qt::WA_Mapped corresponds to 'exposed' in Qt 5, hence a parent window
fully obscured by its children may not have the attribute set.
In that case, go by the attribute of the child widget.

Task-number: QTBUG-33520

Change-Id: Ia08f283172daaa58f58e5636004461b2c48784f7
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Added function returning build information to QLibraryInfo.

Useful for bug reports and test logs.

[ChangeLog][QtCore] QLibraryInfo provides information on how
Qt was built.

Change-Id: I867197fd7d2e10bcdf01a8eb47c9c1e03647e2c1
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>