xF2 Add-on AndyB GPX viewer 2.3

GPX viewer v2.3 changes:

Improved handling of GPX files.

GPX viewer v2.2 changes:

Now compatible with PHP 8.

GPX viewer v2.1 changes:

Now works with XF 2.2 and newer.

GPX viewer v2.0 changes:

Now compatible with PHP 7.4.

GPX viewer v1.9 changes:

Fixed issue where empty variable caused server error.

GPX viewer v1.8 changes:

Added code to prevent malformed GPX attachment data from causing server error.

GPX viewer v1.5 changes:

Now works with PHP 7.2.