* Show splitview icons in the sidebar popup menu to reinforce the connection
and disambiguate the terms "horizontal" & "vertical"
* 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)

Plus two small cleanups I hit upon while working on this:

* Remove unneeded appReady() code in WorkingSetView (also fwiw, it should
have been htmlReady() anyway)
* Fix out of date comments in DefaultMenus
- Tweak splitview icon vertical alignment per @larz0
- Fix JSHint nit: unused local var
Merge pull request #9154 from adobe/pflynn/splitview-menu-icons

Show splitview icons in the sidebar popup menu