OSX: be consistent in using either ~/.config/QtProject or ~/Library/Preferences for ini files

~/.config is a typical place for application-specific config files on Linux (a freedesktop.org standard), so we have followed suit on OS X: qtchooser uses conf files in ~/.config/qtchooser, and Creator has some files there. However QLoggingRegistry has broken rank by expecting qtlogging.ini to be in ~/Library/Preferences.

As a cross-platform developer, it's good for my muscle memory to always go to the same place. And categorized logging (like qtchooser) is a feature for developers rather than end users. And ~/Library/Preferences contains mostly .plist files, not the kind of config files that the user would typically edit by hand. But regular users might have different expectations. QSettings actually does put .plist files there.

It would be better if we were consistent across modules and projects. For now I have made a symlink

~/Library/Preferences/QtProject -> ~/.config/QtProject

(which I suppose doesn't do any harm to keep for the long term, for that matter).