Add support for mandatory instances

[darkvertex]

pyblish-lite implements this by omitting the checkbox when an Instance has .data['optional'] = False, effectively making an instance "mandatory" (in whatever state it was added.)

With this PR, pyblish-qml implements mandatory isntances by greying out the checkbox, just like it's already doing for the "Context" entry in the top left. Clicking on them shows a "Cannot toggle." status prompt.

To show what this looks like, here's a dummy Collector I used:

class Collector(pyblish.api.ContextPlugin):
    order = pyblish.api.CollectorOrder
    def process(self, context):
        instance = context.create_instance('farmer', family='human')
        instance.data['optional'] = False
        instance.data['publish'] = True
        for i in range(3):
            instance = context.create_instance('potato%s' % i, family='vegetable')
            instance.data['optional'] = True
            instance.data['publish'] = False

And here's what this looks like in action:

and for comparison, the same Collector without the PR's changes, in action:

and the same Collector again, with the latest official release of pyblish-lite:

If this was to be merged, it would fix issue #291.

[mottosso]

Haha, omg was this all that had to be done!? :D Well spotted!

[mottosso]

Thanks for this @darkvertex, would it be possible to increment the patch version? That way I can make a release with this new feature.

[darkvertex]

@mottosso Just bumped VERSION_PATCH in version.py

(Is that the only place for the version number info? Let me know if I have to edit elsewhere.)

[mottosso]

That's exactly right, sorry I should have said!

This all looks good to me, thanks again!