Use Atoms Docks API for our panels

[lgeiger]

Atom 1.17 introduced a new docks API (Used by the tree view, GitHub and Linter packages).
We should use it as well. This will allow us better work in conjunction with other packages and makes our panels more flexible.

• Inspector
  We should do this first since it's simpler to implement with less refactorings and we can get rid react-resizable-box then.
• Watch Sidebar

Relevent resources:

• http://blog.atom.io/2017/05/16/atom-1-17.html
• http://flight-manual.atom.io/hacking-atom/sections/package-active-editor-info/
• https://atom.io/docs/api/v1.17.0/Dock

Example PRs from other projects:

• Make the tree view a dock item atom/tree-view#1056
• Use Docks API steelbrain/linter-ui-default#256 and Cleanup dock hiding implementation steelbrain/linter-ui-default#302
• Render Git and GitHub sidebars in docks instead of panels atom/github#664

[BenRussert]

I will start working on this issue. I think we should ship a release before we finish this due to the atom v1.17 dependency. This way holdouts can still get the new result and watch-views!

[lgeiger]

Even more information on the Atom blog: http://blog.atom.io/2017/05/23/docks-deep-dive.html

[lgeiger]

I refactored the Watch Sidebar as a react component so we can easily use it in a dock.
@BenRussert I'd happy to pair with you in the next few days and move our panels to docks.