Add a specific parser for these, which can be accessed as gcoder.S/P

This also reverts to wx.SHAPED for the gviz packing, and specifies a minimum
size with the correct aspect ratio.

The print thread must always respawn the sender thread when ending (the change
introduced by 9c55957e was checking stop_send_thread, which is set to False by
_start_sender, so it never got it). Instead of adding more logic, let's just
always respawn the thread and kill it immediately.

Removing bluetooth serial devices from the default connections because bluetooth serial ports persist on OSX and get in the way of connecting to real printers (very few, if any of which are bluetooth).

By setting printing to false and not restarting the sender thread when the print_thread exits in the disconnect method this should allow printcore to disconnect even if the print thread is running.

In particular this makes it possible to ctrl+c out of prontserve durring an m109 where it previously errored out and hung with the following error:

Can't read from printer (disconnected?) (SerialException): device reports readiness to read but returned no data (device disconnected?)