Abstract template file location logic and enable plugin templating. This class still...

Abstract template file location logic and enable plugin templating.  This class still needs abstraction which is the next task here.  Only possibly contentious change people should look at here is the use of file_exists() to find template files instead of reling on a hard-coded list of files in template.php.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@11727 7612ce4b-ef26-0410-bec9-ea0150e637f0