Blender Git Loki

Don't rebuild package list every draw()

Remove commented code and fix some stuff broken by de-addonification

Hide addons section

Merge branch 'master' of git.blender.org:blender into soc-2017-package_manager

Move package manager code out of addon

Code from addon repository: https://developer.blender.org/diffusion/BPMA/

Merge branch 'master' of git.blender.org:blender into soc-2017-package_manager

Add a tab to user preferences

Not sure if this is fully correct; getting crashes with non-factory
settings.