Long ago, the menu panel in was a customizable area that users could drag things into.
That changed back around 2017 in bug 1354117 when the Photon redesign was built. The
menu panel become a static menu, but we also made it possible to permanently move things
to the overflow panel of the nav-bar.
It looks like we never updated the area type constant from referring to the old menu panel
though, so it's "TYPE_MENU_PANEL", and registering a node for it happens with
registerMenuPanel. This patch changes to constant to TYPE_PANEL and updates the registration
method to registerPanelNode.
I a check around the codebase as well as GitHub looking to see if there were any
system add-ons or experimental WebExtensions that rely on TYPE_MENU_PANEL / registerMenuPanel,
but I couldn't find any.
Differential Revision: https://phabricator.services.mozilla.com/D161078
c759904dae