Grey out multi-Clear Quest Filepath

I think we should have Etc->Clear Quest Filepath grey out after clicking "Yes" on the Clear Quest Path msgbox, because there is no point in clearing it twice.

The msgbox is created by onClearQuestFilepath() in zq_misc.cpp

etc_menu is a static NewMenu declared in zquest.cpp

The actual clearing is done by ZQ_ClearQuestPath() in zquest.cpp

filepath is a char* pointing to an empty string after a call to it completes.

Here is an example of how to grey out a menu option:

Code:

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                edit_menu.disable_uid(MENUID_EDIT_UNDO, !Map.CanUndo());
                edit_menu.disable_uid(MENUID_EDIT_REDO, !Map.CanRedo());

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