fix Issue #8953- add Extension Manager support for localized extension package metadata.
update localization example for reference
update localization readme for reference
as requested per code review
changes per code review
Merge remote-tracking branch 'origin/master' into bchin/localize-package-metadata
per code review, overlay first by shortlang, then by lang.
Merge remote-tracking branch 'origin' into bchin/localize-package-metadata
whitelisting properties to overlay; remove keyword concatenation with root language
add top-level key to package.json to avoid collisions
changes per code review
adding unit tests
* 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* Fix out of date comments in DefaultMenus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
Initial commit.
Tweaked pane-header lines.
Updated by ALF automation.
changes as requested per code review
Further consolidate duplicated code in a unit test
Merge pull request #8987 from adobe/bchin/localize-package-metadata

fix Issue #8953- "Localizing package metadata in Extension Manager"