[WIP] Improve package management experience - do not merge

This branch is an exploration of one way to make some things better about the package management experience. It's very hacky/WIP, and was an excuse to get back into package development so don't look here for top end code examples... yet.

Here are some frustrations I have with settings-view:

- Packages are listed in a small part of the sidebar, and it's hard to scan through them
- User installed packages, core packages and dev packages are all listed together
- All interesting package actions (uninstall, enable/disable, informational tasks) are only available from the full settings pane
- Interesting metadata that can help a user assess a package's quality and maturity is not always easily available

Here are some ways this branch addresses (or will address) these issues:

- give the concept of managing packages & themes its own set of panes, and keep it separate from new package discovery, search, and installation
- Show packages in lists with action buttons and metadata similar to the package cards at https://atom.io/packages/list
- group packages by installed, core, dev and provide counts when filtering
- provide an api and avatar cache so that we can show metadata from the API without ruining the offline experience

![settings-view-480](https://cloud.githubusercontent.com/assets/1476/5151055/0206b02a-718e-11e4-81b3-c72a88720678.gif)

The current state of this branch is rough ([and depends on an unfinished PR in apm](https://github.com/atom/apm/pull/216))- I chose to sketch in code rather than approach the UX more formally. I think that whether this code gets merged or not the process & discussion could help us get the settings view right.

Along those lines I want to hear what folks think about this - of course anyone watching this repo, but let me specifically call out @simurai and @atom/core for comments on the direction.