bank: add support for browsing frameworks

Darwin platforms are using frameworks to embed libraries. Some of them
like iOS and tvOS even forbid the use of dynamic libraries that are not
wrapped into a framework.

Since frameworks can contain various things (headers, resources, etc),
we don't necessarily want to browse their whole tree if their structure
is well-defined. In our case, since plugins are bundles and not dylibs,
there's not linking and there's no point in supporting versionning so we
can expect the plugin's dynamic library in the root of the framework,
named like the framework without the .framework part.

Since MacOSX can also make use of frameworks, this is enabled for all
darwin platforms, though it might not use them.