Grey out multi-Clear Quest Filepath

[Asuna Yuuki Nagato]

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.

[Asuna Yuuki Nagato]

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

[Asuna Yuuki Nagato]

etc_menu is a static NewMenu declared in zquest.cpp

[Asuna Yuuki Nagato]

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.

[Asuna Yuuki Nagato]

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

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