[development] make a hook_theme_variables that extends the job of _phptemplate_variables

[Robert Douglass]

one of the most useful things about PHPTemplate is the template.php 
file. One of the most useful things about template.php is the 
_phptemplate_variables. Why don't we extend this to be a module hook 
that lets module access the variable stack prior to being sent to any 
template? This would solve the problem that I ran into in 
page_title.module where the module is dependent on an alteration to the 
_phptemplate_variables function.