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* Fix out of date comments in DefaultMenus
No they shouldn't. All the icons in Brackets are flat except for the ones in the sidebar. Side bar is the only place we use drop shadows heavily on the UI e.g. files and icons.