The original app_id hard-coded replacement in scripts and hooks is replaced by
the use in app scripts directly of the given app_id argument (wip #126).
It's now the app maintainer responsability to make use of it in a way to
prevent colisions with other instances.
It passes the current app id to the app install/upgrade/remove scripts
as the last argument (wip #126).
The hook category has also been revisited to move some app specific parts
away from there. The action hook_check has been removed accordingly.
