add new arg for menu.build: sub_menu

[Nimlar]

Allow to set the freedesktop generated menu in its own entry.

usage example in rc.lua:

mymainmenu = freedesktop.menu.build({
        before = { menu_awesome },
        after =  { menu_terminal },
        sub_menu = "freedesktop"
    })

[Sorky]

Nice

[Sorky]

Works perfectly - FWIW - I approve

Personally I'd adjust the commenting...

'-' -- Add items to menu
'+' -- Add menu item to hold the generated menu
' ' if sub_menu then
' ' result = {{sub_menu, result}}
' ' end
'+'
'+' -- Add items to menu

[Nimlar]

Thanks.