This adds a the start of a sidebar, as sort of discussed in the spec. It's pretty obviously inspired by BigBoard...

Depends on bug 581769 and bug 581771.

Current state:

    - It exists, it has struts to keep your windows out of the way

    - Expandable/collapsable, like BigBoard, except with sexier animations :)

    - Current widgets: expand/collapse toggle, clock, app launcher. These
      are all just proof-of-concept. The app launcher just reuses the actors
      from appDisplay.js (which requires putting them onto a black
      background that clashes with the rest of the sidebar, because
      otherwise you can't read the white text, etc). We need to abstract out
      some code there...

    - In collapsed view, widgets default to showing a rotated title, as in
      BigBoard, and the expanded view pops out on hover. (You can see this
      with the app launcher widget.) However, alternatively, a widget can
      provide a separate collapsed view; the clock demonstrates this, by
      drawing an analog clock in collapsed view (which is not really that
      readable, but whatever). And really, I want the app launcher to
      display mini icons in the collapsed view, but I left it the way it
      is now to demonstrate/test pop-out. Also, probably we want things to
      be able to both have a separate collapsed view *and* do pop-out.
      (Eg, the clock could pop out a full date/time display, or maybe even
      the calendar.)

    - The current set of widgets is hardcoded in a variable in sidebar.js,
      but everything is set up for the idea of arbitrary widgets:

          const default_widgets = [
              "imports.ui.widget.ClockWidget",
              "imports.ui.widget.AppsWidget"
          ];

      Sidebar.addWidget() takes a string, verifies that it has the right
      form and refers to a constructor function for a class that inherits
      from imports.ui.widget.Widget, and then invokes that constructor to
      create the widget, puts it in a WidgetBox, and adds it to the sidebar.
      WidgetBox enforces the correct size and clipping on the widget, and
      all calls into methods provided by the widget are wrapped in try/catch
      blocks to protect the sidebar against bugs in widget code. Obviously
      none of this is actually secure against malicious code, but it should
      do an ok job with merely bad code.

    - Hides itself when going into the overlay, shows again on exit. The
      spec suggests that it's actually supposed to *turn into* the overlay's
      sidebar, though that may not really make sense graphically. Also,
      it probably ought to animate that transition... Also, there's supposed
      to be a hide/show sidebar item in the user menu, but there isn't yet.

I guess next up is figuring how much the overall design should change, what additional/different functionality is needed, and making more widgets...