easier way to call functions in plugins  fixes #5

* don't enforce plugins to explicitly list callable functions
* make all plugin functions callable, handler or not
* make sure plugin pages can also display error/success boxes

this fixes a bug where sometimes render_last_page tried to call a
function that was in a plugin's scope, and would result in
http 500 internal server error
Don't go back to pages that can't display success/error boxes
show success/info/error/.. boxes on page_light too
form for product related events