XML export: fix system notice

Use of incorrect variable caused invalid XML file generation due to PHP
system notice being added to output
XML import: use require_api() instead of require_once()
XML import: fix language string

Use a variable in the language string to display the project name, to
allow flexible positioning of the project name in other locales.
XML import: fix 'Creating default object from empty value'
XML import: fix page layout
XML import: fix system notices
XML import: fix PHP notices
XML import: fix import page layout
XML import/export plugin fixes

This resolves a number of warnings and notices with the plugin when
strict error reporting.

As far as I can tell, the fixes mentioned in issue #12013 after dhx
committed the changes (84017535f8718685d755d58af7a39d80f52ffca8), i.e.
bugnotes are not needed:
- ~28391 by schoppi71: don't think this is a bug
- ~28855 by ninguno: already included in dhx's commit