Handle PERM_EXPORT in LLPermissions.

This moves all export test code to where it belongs:
in LLPermissions.

Added LLPermissions::allowExportBy, next to
allowModifyBy, allowCopyBy and allowMoveBy.
Then changed all code to use this call.

Because LLPermissions is part of llinventory, I had
to add a proxy for LFSimFeatureHandler.
Added a new class LFSimFeatureHandlerInterface that can be
used by LLPermissions to check simulator features by
accessing the LFSimFeatureHandler singleton.

Several parts of the code ignored the PERM_EXPORT but
and still did demand that things are full perm next
to being a creator. This has now changed the export
rules are the same for everything as they were for mesh:
you need to be the owner and the creator for every element
that is exported (not just the root prim, of course).

Export rules can now be easily made a function on
simulator features. If different rules apply for different
types (wearables, objects, mesh etc) then an extra variable
indicating the type will have to be passed though.

indra/newview/llviewermenu.cpp

@@ -2895,15 +2895,7 @@ class LLObjectEnableExport : public view_listener_t
 				const LLSD& userdata;
 				virtual bool apply(LLSelectNode* node)
 				{
-					// Note: the actual permission checking algorithm depends on the grid TOS and must be
-					// performed for each prim and texture. This is done later in llviewerobjectbackup.cpp.
-					// This means that even if the item is enabled in the menu, the export may fail should
-					// the permissions not be met for each exported asset. The permissions check below
-					// therefore only corresponds to the minimal permissions requirement common to all grids.
-					LLPermissions *item_permissions = node->mPermissions;
-					return (gAgent.getID() == item_permissions->getOwner() &&
-							(gAgent.getID() == item_permissions->getCreator() ||
-							 (item_permissions->getMaskOwner() & PERM_ITEM_UNRESTRICTED) == PERM_ITEM_UNRESTRICTED));
+					return node->mPermissions->allowExportBy(gAgent.getID());
 				}
 			};
 			ff * the_ff = new ff(userdata);