[Development] Behaviour change of opacity property in QtQuick 2.0

[Nils Jeisecke]

> Apparently, it was intentional: QTBUG-19193, and commit
> 9bf96613c1be7d4df7132a9c3c9ac1a8740f698f. It would be nice to know what
> the arguments for changing the behaviour were...

Ok, thanks. I should have checked the git log. Sorry.

However this change makes QtQuick porting even more complicated
because it can be quite hard to figure out why a mousearea that is
below some invisible popup (visible but opacity==0) no longer reacts
to clicks but to hover events...

Nils