Introduced three new events:
* CONNECTING - fired just before starting the connection process
* DISCONNECTING - fired just before starting the (active) disconnection
process
* PRINTER_STATE_CHANGED - fired every time the printer state changes
Also introduced new GCODE script beforePrinterDisconnected, which will
get sent just before the printer gets (actively) disconnected. Also enabled
communication object to wait for the send queue to be emptied before
closing it, in order to allow sending all lines from the disconnect script.
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