Refactor MenuItemMorph to support use of a FormSet and apply that to CmdCommand

[Rinzwind]

This pull request refactors MenuItemMorph to support use of a FormSet for the icon and applies that to the CmdCommand hierarchy. In the CmdCommand hierarchy, the overrides of #defaultMenuIcon are refactored to override #defaultMenuIconName instead.

[Rinzwind]

I converted this to a draft. Commit 02f48dcb43a0cc8d introduces a bug in #setUpIconForMenuItem: where the argument, despite the variable name ‘aMenuItemMorph’, can also be a PragmaMenuAndShortcutRegistration, which does not understand #iconFormSet:.

[Rinzwind]

I fixed the bug referred to in my previous message by first refactoring PluggableMenuItemSpec and PragmaMenuAndShortcutRegistrationItem as well.

[Ducasse]

@jecisc can you check it? Tx

[Rinzwind]

@jecisc Could I ask for an update on the (re)review? If you’re not available, it might be better to let someone else take over?