change naming convention for template source files

rename *.cpp => *_tpl_p.h
otherwise, syncqt doesn't generate code to install them, which makes the
"regular" private headers which include the template code useless.

Change-Id: I7f3a34bf442fb003bcb7ef96dd4f88d47bd6598b
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>