[Development] Modify QLibraryInfo to support any default location of qt.conf

[Welbourne Edward]

>>> 2. Also lookup qt.conf in all of QLibraryInfo::libraryPaths() (this is
>>> how we got the plugins found)
>>
>> Potentially problematic if plugins might install a qt.conf that isn't
>> what your application needs.
>
> I have full control of the content of my application bundle and the
> libraryPath list is „cleaned up“ before instantiating QApplication
> to only contain paths into the bundle directory. So at least for me
> this could happen. Can you please give an example where this could
> cause trouble?

I was imagining you meant third-party plugins that might be in random
places.  I take it you actually mean plugins that are part of your
package, so now I see why this won't be a problem.

> I think one of the three possible solutions is enough.

Ah - I see - your list was of candidates (of which to implement one) in
preference order, rather than of places an application can find qt.conf,
in search order.  Then I can see why you would prefer the others over
an environment variable.

See Ossi's mail, though, for the
https://bugreports.qt.io/browse/QTBUG-15234
solution, that hopefully makes reconfiguring the qt.conf search path
redundant,

	Eddy.