Use gettext algorithm to determine locale for activity.linfo

When activity bundles are loaded, the activity name and summary are pulled from an activity.linfo file that is associated with locale. However, sometimes locale, which uses $LANG has been superceded by $LANGUAGE as set in extensions/cpsection/languages/model.py. This patch uses the same algorithm as gettext.py to find activity.linfo

This problem is seen on the .AU systems which use en_AU for locale and en_GB for LANGUAGE.
Merge branch 'fix-locale-3' of https://github.com/walterbender/sugar-toolkit-gtk3 into fix-locale-3