Reviewed-by: hjk

Adds a 'type' attribute to the 'dependency' tag, with possible values
'required' and 'optional'.
Optional dependencies may not be linked against. You'll need to use the
new dynamic methods in plugin manager (retrieving objects and calling
methods by name) if you want to access functionality of optional
dependencies.

... instead of running a sed script over a lot of files on each
version increment.