Transfer box customization

This work is based on the work by @CrosRoad95 in pull request #171

grafik
You are able to hide and show the transfer box on clients.
You can hide the transfer box from server-side before the client starts resources to not show the download window at all.
You are supposed to use a resource with higher download priority to show a custom transfer box.
You will only receive events for the resource download transfer box.
The map download transfer box does not emit these events and can not be hidden with the scripting functions.
Players can force the download window to always appear regardless of the script provided setting through settings.

New server functions

These functions are global. That means they apply for every player.

bool setTransferBoxVisible(bool visible)
bool isTransferBoxVisible() -- returns false only if the server wants the transfer box hidden for players

New client functions

bool setTransferBoxVisible(bool visible)
bool isTransferBoxVisible() -- returns true if transfer box should be visible (if a download is running), it does not represent the value set by setTransferBoxVisible
bool isTransferBoxAlwaysVisible() -- returns true if the user enabled the main menu setting to always display it (cvar)

New client events

onClientResourceFileDownload(resource-pointer resource, string fileName, number fileSize, string state)
-- state can be "queued" or "finished" or "failed"
onClientTransferBoxProgressChange(number downloadedBytes, number downloadTotalBytes)
onClientTransferBoxVisibilityChange(bool isVisible)