Naming: replaced notification/receiver with signal/slot.

The names "signal" and "slot" communicate the respective direction of event
transmission more clearly.
Extracted request function constructor for reuse, created "defChannel"
skeleton.
Outline of channel implementation.
Implementations of channel methods.
BUG FIX: replaced 'shift' with 'unshift'.
Use the 'makeChannel' function of WsChannel.js to create a channel.
Define and use a channel for the file upload.
Return a result when closing the channel, fixed naming issues.

As closing/aborting a channel might be a no-op if the channel is not open,
a corresponding result will be returned by the respective channel methods:

A return value of 'true' indicates that the channel is open and a request
to close/abort the channel will be sent.

A return value of 'false' (= no-op) indicates that no request will be sent
and, consequently, the specified response handler will NOT be invoked.
Refactoring: unified channel read/write operations.

Extracted common code of read and write operations to data transmission
function that is invoked with the respective transmission direction as an
argument.
Refactoring: unified channel close operations.

Extracted common code of close and abort operations to a function that
closes the channel.