Improve Gemini PR, Passport, and Procat support

[benoit-pierre]

Add an _iter_packets(packets_size) helper to SerialStenotypeBase: yield packets of packets_size bytes until the machine is stopped. To workaround the fact that the Toshiba Bluetooth stack on Windows does not correctly handle the read timeout setting (returning immediately if some data is already available):

• the effective timeout is re-configured to timeout/packet_size
• multiple reads are done (until a packet is complete)
• an incomplete packet will only be discarded if one of those reads return no data (but not on short read)

Update the Gemini PR code to use this new method and reject invalid packets. Fix #846.

Note: the Procat code should probably be updated to do the same.

[benoit-pierre]

And the Passport code should probably do the same as the Tx Bolt, i.e.:

raw = self.serial_port.read(max(1, self.serial_port.inWaiting()))

[benoit-pierre]

Updated to improve support for the following machines:

• Procat: use the new method for getting packets, discard invalid packets
• Passport: do not hog the CPU constantly trying to read 0 bytes of data from the serial port when no data is available....