Make navigation columns resizable

This adds resizable column functionality to navigation columns. The changes are as follows:

Makes the `drag-handle` component work on left and right sides, as well as with nested split-view panels. It has a `minWidth` option to prevent columns from being reszed too small.

Adds a `draggableColumn` view which wraps a `resizable-column` component and a `drag-handle` component. This allows for the “drag action” to be easily broadcast and handled by the application route (this is currently handled by modifying the `isDragging` attribute on the application controller)

**Before**

    // some-template.hbs
    {{#resizable-column width=navWidth}}
      content
    {{/resizable-column}}

    {{drag-handle positionRight=navWidth isDragging=isDragging}}


    // some-template-controller.js
    needs: ['application'],
    isDragging: Ember.computed.alias('controllers.application.isDragging')

**After**

    // some-template.hbs
    {{#view "draggableColumn" side="right" width=navWidth}}
      content
    {{/view}}