Isolate pocketmine network calls

[Muqsit]

NetherGamesMC maintains the most popular fork but goes through a strenuous process to keep up with library during network-heavy updates like v4.7.1. sendDataPacket() and other pocketmine\network calls are directly invoked in various parts of InvMenu, which is largely the sole cause of the issue.

Goal is to refactor the library and isolate these calls to a utility class, preferably one that plugins can override, e.g. InvMenuHandler::$network = new MyInvMenuNetwork();, and dually reduce the maintenance burden in forks.