GrampsLocale: Suppress double printing of log messages

Grampsapp.py sets a global handler, so install one only during the first init and remove it when done.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@21644 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
Plugins: Limit addon translators

Only get an addon translator if the plugin has a locale directory
Prevents pointless warnings, esp. when the translations are in the
main Gramps message file.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@21645 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
GrampsLocale: Code cleanup

And make sure that setting lang on the GrampsLocale constructor
overrides the languages from the first instance.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@21646 4ae1f11a-8b86-4847-b8af-ab372f36d1fd