This fixes a bug where pinning a Private Browsing window to Taskbar with the windows context menu item pins regular Firefox instead. This happens because Windows cannot find an appropriate shortcut (presumably by looking for one with the right AUMID), and ends up creating its own instead -- with almost entirely incorrect metadata.
With this, we'll be creating one at a few points (if it doesn't already exist):
* Installer - for new installs
* Post-Update - to make sure we get one when people update to the first version where we pref this on
* Startup idle - for zip installs, and to handle any other possible case where the shortcut doesn't exist
Until we enable separation of Private Windows by default I've pref'ed off this behaviour (otherwise a user may pin the shortcut to the Taskbar, but the app will launch into a different icon).
Differential Revision: https://phabricator.services.mozilla.com/D147702
03c5d6d051