Show splitview icons in the sidebar popup menu

@larz0 what do you think of this UI tweak?  Showing splitview icons in the sidebar popup menu helps reinforce their meaning, and also disambiguates the terms "horizontal" & "vertical" (i.e. does "horizontal" mean two panes arranged horizontally, or two panes arranged in a vertical stack with a horizontal divide running between them?)

And a few small technical cleanups in here:
(the first is directly related to the menu icons; the others are side things I just happened to hit upon while working on this)

* Don't programmatically compute spritesheet offset in SidebarView.js, and avoid using `background-position-y` (which apparently is not W3C and isn't supported in Firefox)
* Remove unneeded `appReady()` listener in WorkingSetView (also fwiw, it could have been just `htmlReady()`)
* Fix out of date comments in DefaultMenus