[Development] "Native event" filter

[David Faure]

QWindow and QWidget have a virtual method nativeEvent(), but this only works 
for window-specific or widget-specific event handling.

Looking at the KDE code, I see a rather wide usage of process-level handling 
of native events.

http://lxr.kde.org/ident?i=installX11EventFilter shows about 30 uses of that.

>From idle detection to desktop-wide clipboard handling, color picking, RANDR 
event handling, and so on.
This was all done via QApplication::x11EventFilter(), which doesn't seem to 
have a replacement in Qt5 yet.

1) any objections against adding this to Qt 5? (for 5.1 if you want, assuming 
it can be done without new virtual methods)

2) what could the API look like? Ideally similar to QEvent event filters, so

QGuiApplication::installEventFilter(Receiver *) ?

The problem is the type of Receiver. QObject can't be used here, since there 
is no nativeEvent() there. So it's either QWidget (but this tends to make 
people write 'fake widgets' just to receive these events), or a custom class, 
something like:

class QNativeEventFilter
{
    virtual bool nativeEvent(const QByteArray &eventType, void *message, long 
*result) // same API as in QWindow/QWidget.
};

(not sure if it should be a QObject for easier memory management)

-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Sponsored by Nokia to work on KDE, incl. KDE Frameworks 5