Unregister/clear should be more powerful

[Timoses]

venu#unregisterAll should be renamed to venu#clear (it actually clears/deletes all menu items).

Further, more argument types should be supported in venu#clear(arg). arg may be:

• a menu/submenu
• name (string) of a menu/submenu
• () empty -> clear all (like venu#unregisterAll currently)