Bug fixes and various changes

This pull request contains a range of changes:

Updated translations and fixed some issues loading translations
Updated default death prefix symbol to † instead of +
Added field validation for the birth and death prefix fields in the control panel settings
Added new option 'Treatment of source individuals' to the control panel so the admin can set a default value
Added message if XREF is loaded from URL and individual is added to list of starting persons

Resolves #347, #359

Also partly resolves #361 however conversation there is ongoing

Neriderc / GVExport