`Downloader`: Asynchronous downloading to memory, apply to release checker

[Vankata453]

Transfer is now a base class for two new derived classes: FileTransfer and StringTransfer. FileTransfer has the same functionality which Transfer had before - it writes received data to a file. StringTransfer, however, appends received data to a referenced string. These additions are also supported by WASM (NOTE: Needs to be tested).

A StringTransfer, which is requested by calling Downloader::request_string_download(), is now used for downloading the "supertux-versioninfo" file, used by the in-game release checker.

Additionally:

• WASM now supports downloaders, other than the AddonManager one, by providing a pointer to the current Downloader (NOTE: Needs to be tested).
• Support for parent TransferStatusLists has been added to all Downloader functions.
• ReaderDocument now contains the from_string() static function, which is a convenience wrapper around from_stream(), where a stream, which only has the given string in its buffer, is provided.

Closes #2627.