Sort instances by label in pyblish gui

[iLLiCiTiT]

Changes

• context item is not under any group item - possible since we don't have artist view anymore
• items in instance view are sorted except context item which is always at the top
• this change does not affect order of processing

Resolves #696

[mkolar]

This is not great actually. It also sorts the plugins on the right side, which makes is super confusing to see what's going on, considering they are processed in different order, that how they are displayed.

The instances mismatch between visual ordering and processing is only a bit annoying, so I'd accept that, but the plugins are not usable.

[iLLiCiTiT]

It also sorts the plugins on the right side, which makes is super confusing to see what's going on, considering they are processed in different order, that how they are displayed.

Good spot I actually didn't realize that.

[mkolar]

Much better. thanks