Bugzilla

As a follow-up on bug 1546309, four of the appmenu submenus need to display nested account/folder items and submenus, as the `folder-menupopup` custom element does for non-appmenu menus.  Adapt the `folder-menupopup` code so it can be used for the appmenu.  

This was deferred to a follow-up because the folder-menupopup was not yet de-xbl'd yet at the time, and due to the size of the bug 1546309.

This will need to land for both 68 beta and trunk.

One approach would be to make a custom element for the appmenu submenus that shares some of the code with the folder-menupopup custom element (likely via a mixin, since I don't think the appmenu CE will need or want to extend the MozElements.MozMenuPopup like folder-menupopup does).

As a follow-up on bug 1546309, four of the appmenu submenus need to display nested account/folder items and submenus. These are: 

- File > Get New Messages For
- Go > Folder
- Message > Move To
- Message > Copy To

The `folder-menupopup` custom element handles this for non-appmenu menus.  Adapt the `folder-menupopup` code so it can be used for the appmenu.  

This was deferred to a follow-up because the folder-menupopup was not yet de-xbl'd yet at the time, and due to the size of the bug 1546309.

This will need to land for both 68 beta and 69 trunk.

One approach would be to make a custom element for the appmenu submenus that shares some of the code with the folder-menupopup custom element (likely via a mixin, since I don't think the appmenu CE will need or want to extend the MozElements.MozMenuPopup like folder-menupopup does).