mod_base: Add {maxage} option to controller_template

Enabling client-side caching of dynamic pages.
mod_base: z_set_validator_postback no longer throws an error when re-binding a validator postback to an element which had already one.
doc: Add note about new (undocumented) mod_export module to NEWS file
i18n: Remove .pot files from the repository

They are generated by mod_translation, so can easily be re-created.