[mtasc] Support of ContextMenu class

[Nicolas Cannasse]

> Hello mtasc,
>
>   Will MTASC support 'ContextMenu' class in a future?
>
>   Currently it throws an error when compiling *.as files:
>   "type error: class not found ContextMenu".

ContextMenu and ContextMenuItem have been added on the CVS.
If you want to use them with current release you can simply copy them from
your  MM install / en / FirstRun / Classes to the MTASC "std" directory.

Nicolas Cannasse