Targets workflow.

[tokejepsen]

Motivation

This PR is to facilitate the targets workflow that was introduced in https://github.com/pyblish/pyblish-base/releases/tag/1.5.0.

Implementation

The implementation for pyblish-qml is done so you can exclusively target plugins. For a demonstration you can execute:

$ python -m pyblish_qml --demo --targets studio

This will only load two plugins that are targeted directly for studio.
To get all plugins you can target both default and studio:

$ python -m pyblish_qml --demo --targets default studio

You can also specify the targets when showing pyblish-qml from a host:

import pyblish_qml
pyblish_qml.show(targets=["studio"])

Because the matching is using pyblish.api.plugins_by_targets its important to get pyblish/pyblish-base#321 merged first.

Lastly I'm not sure whether there is better way than passing the targets argument all the way through to the controller?

[tokejepsen]

Latest update facilitates registering new targets in the host.

import pyblish_qml
import pyblish.api

pyblish.api.register_target("studio")
pyblish_qml.show()

[tokejepsen]

This has been used for a week in production. Proceeding to merging.