[WIP] Modularization: Switch to 'oni-core-window-manager'

[bryphe]

This factors out the window management to a separate npm module: oni-core-window-manager:

https://github.com/onivim/oni-core-window-manager

Some remaining work:

• There is a bug where the dock 'sidebar' isn't showing - some of the styles are in common.less and need to be ported over
• The 'error' component is specialized for Oni - need to add a hook where we can inject a custom error component.