Bug 6928: Windows: If LANG not equal en_GB Graphic Reports

in some cases produce empty PDF sheets or force graphviz to crash

Problem was due to calling localized float() when parsing a C-locale xml file

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@22705 4ae1f11a-8b86-4847-b8af-ab372f36d1fd