Formspec: Show a player inventory using core.show_formspec (#15963)

'core.show_formspec' now shows and updates the inventory formspec as if
it was opened using the hotkey on client-side.

src/network/clientpackethandler.cpp

@@ -902,6 +902,7 @@ void Client::handleCommand_InventoryFormSpec(NetworkPacket* pkt)
 
 	// Store formspec in LocalPlayer
 	player->inventory_formspec = pkt->readLongString();
+	player->inventory_formspec_override.clear();
 }
 
 void Client::handleCommand_DetachedInventory(NetworkPacket* pkt)

src/network/serveropcodes.cpp

@@ -178,6 +178,7 @@ const ClientCommandFactory clientCommandFactoryTable[TOCLIENT_NUM_MSG_TYPES] =
 	{ "TOCLIENT_STOP_SOUND",               0, true }, // 0x40
 	{ "TOCLIENT_PRIVILEGES",               0, true }, // 0x41
 	{ "TOCLIENT_INVENTORY_FORMSPEC",       0, true }, // 0x42
+	// ^ `channel` MUST be the same as TOCLIENT_SHOW_FORMSPEC
 	{ "TOCLIENT_DETACHED_INVENTORY",       0, true }, // 0x43
 	{ "TOCLIENT_SHOW_FORMSPEC",            0, true }, // 0x44
 	{ "TOCLIENT_MOVEMENT",                 0, true }, // 0x45