Add function to find a TopMenu Item

[Ilusion9]

This can be used to change the logic of TopMenu items (find item -> remove -> create again) registered by base plugins (basecommands for example). I will use this to change the logic for sm_who (instead of flags, I will display their groups) so I don't have to modify basecommands whenever its updated.

[KyleSanderson]

It looks like you may have forgotten to include the header change

[peace-maker]

The function needs to be added to the public ITopMenu interface too. https://github.com/alliedmodders/sourcemod/blob/master/public/extensions/ITopMenus.h