add support for adding a new plugin in the UI

[hannesdelbeke]

plugin selection righ now just loads registered plugins
add support for adding a new plugin in the UI
might tie in with pyblish/pyblish-base#330
also support adding tthe same plug multiple times
example:
a collector that runs on meshes with prefix AB_ and CD_ could reuse a collector that collects bashed on prefix , instead of adapting said collector to support multiple prefixes