Add option to use SmartyBC instead of Smarty class

[benkeen]

Needed for backward compatibility with users using {php} in their Smarty content. See comments in #181.

Note: will need to review all modules and check for references there too.

• They'll be linked to below.
• I'll update those modules once 3.0.1 is released. For backward compatibility with FT3.0.0, each module will check for the existence of the Core::useSmartyBC method. If it doesn't exist, it'll just use the default Smarty class.

[benkeen]

Feature documented here under $g_use_smarty_bc:
https://docs.formtools.org/userdoc/getting_started/config_file/