Add "displayStr" param to native menu APIs

**NOTE: This is an API breaking change!**

Add a new `displayStr` parameter to the `addMenuItem()` API added last sprint. This is required so we can pass localized shortcut strings from JavaScript to the native menus. On Windows, this is needed for languages like German that show "Strg-O" instead of "Ctrl-O". On Mac, this parameter is ignored since the modifier keys are always shown as symbols.

This pull requests also adds a new `setMenuItemShortcut()` API. You should be able to guess what it is used for :-)

There is a corresponding brackets pull request to update the JavaScript code for the API change.