BUG: Ensure packaged Slicer.app MacOSX application find its Qt plugins

The assumption that an installed Slicer.app bundle would find and load the
file "Slicer.app/Contents/Resources/qt.conf" is wrong.

Since within "Application/SlicerApp/Main.cxx", both methods (a) "QSettings()"
and (b) "QApplication::setStyle()" are called before the QApplication
is instantiated, the "QLibraryInfoPrivate::findConfiguration()" was
not calling "CFBundleGetMainBundle()" and couldn't locate the "qt.conf"
file relatively to the Slicer.app directory.

As a consequence:
(1) the correct plugin couldn't be loaded and
(2) trying to run an installed Slicer.app in the environment where it
was build was failing because "QLibraryInfo::location()" was defaulting
to the plugin location associated with the build tree.

Since calling "QSettings()" and "QApplication::setStyle()" is not possible.
This commit implements a different strategy. It adds the "qt.conf" file to
the resource system (See [1] for details about this).

[1] http://doc.qt.io/qt-4.8/qt-conf.html

This approach will ensure:

(1) the installed Slicer.app finds its plugin: "QLibraryInfoPrivate::findConfiguration()
will check if the resource exists independently of the QCoreApplication
instance.

(2) the application in the build tree still work. The plugin will be found
because of the path hardcoded in "QLibraryInfo::location()"

References:

• https://forum.qt.io/topic/50305/qt-5-3-and-5-4-mac-application-crashes-on-startup-after-using-macdeployqt-app-unable-to-load-plugin-cocoa/7

Co-authored-by: Alexis Girault <alexis.girault@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24465 3bd1e089-480b-0410-8dfb-8563597acbee