In the regressing change part of Bug 1755181 (https://hg.mozilla.org/mozilla-central/rev/00c77cbfd73a),
SidebarUI observes "intl:app-locales-changed" and calls `SidebarUI.hide` and `SidebarUI._show(this.lastOpenedId)`
if the sidebar was open, that is enough to correctly refresh sidebar panels part of the browser but it is missing
the additional call to `SidebarUI._loadSidebarExtension(commandID)` that both `SidebarUI.show` and `SidebarUI.showInitially`
call once `SidebarUI._show(...)` promise is resolved.
This patch includes a new test case to explicitly cover the behavior expected on extension sidebar panels
(added to browser_sidebar_app_locale_changed.js along with the tests covering the expected behavior for
the browser integrated sidebars) and replaces the call to `SidebarUI._show(...)` with a call to `SidebarUI.showInitially`
(mainly because the associated jsdoc inline comments explicitly states that showInitially is intended to be used
when the sidebar is opened automatically and not triggered by a user interaction with the sidebar UI, which seems
to fit better then show for this particular case).
Differential Revision: https://phabricator.services.mozilla.com/D155575
a5fa70b662