Improve the new installed languages api after discussion with dfaure

Change name from allLanguagesInstalledList() to a more natural
installedLanguages(). Oh the indignity of having one's English corrected by
a Frenchman :-)

Move the en_US entry.desktop file from kdebase/kcontrol/locale/default to a
more logical and obvious kdelibs/kdecore/localization. This means the revised
unit test will also always work. Move the all_languages.desktop there as well.

The kdebase delete will follow shortly.

And while I'm there, fix the problem steveire found with KLocale constructor
being too fussy about the case of the country code and lack of documentation
for it.