Enable TextPad to combine .syn files

[wstein]

The basic issue this addresses is best explained with an example:

The PHP5Drupal6.syn file might be very small and simply say: "Use PHP5.syn for syntax, and add Drupal6.syn as additional keywords".
The PHP5.syn file would be maintained and updated to be correct for any *.php file. Programmers (like me) who are using Drupal on PHP5, would opt for the "PHP5Drupal6.syn" mix file. Other programmers (or other projects) that use straight PHP5 would opt for the PHP5.syn file.
There could even be another mix file, "PHP5Wordpress.syn" which would combine the basic syntac from the PHP5.syn file with a "Wordpress.syn" list of additional keywords.