[Review Only] Inline Bezier Curve Editor

This implements the [**Quick Edit: CSS Cubic Bezier Trello Card**](https://trello.com/c/5EPJdO1q/838-2-quick-edit-css-cubic-bezier) *except* for Unit Tests.

I marked this as "Review Only" because the commits need to be squashed before merging. I submitted this pull request before squashing to have the history available until the code review is complete.

**Testing:**

CSS Timing Function values specified by cubic-bezier() function or pre-defined values (linear, ease, ease-in, ease-out, ease-in-out) can be visually manipulated as follows:

1. Dragging an endpoint. Code updates on drop.
2. Clicking in grid to move nearest endpoint.
3. Clicking endpoint (to give it selection), using keyboard navigation arrows. Use Shift key to accelerate.

**Other Cases to Test**

- multiple inline bezier curve editor instances
- try to move outside grid with mouse (both holding down mouse and releasing before re-entry) or with keyboard
- moving mouse around grid displays Time and Progression percentages.
- editing pre-defined values shows actual curve, and editing converts it to cubic-bezier() format.

**Known Issues**

1. <del>inline editor does not update when code is edited in page</del> Fixed
2. <del>Ctrl/Cmd-E does not close inline editor</del> Fixed
3. <del>No Close button</del> Fixed
4. <del>Code is not updated when dragging mouse</del> Fixed
5. Special cases (e.g. 'linear') are not updated after editing