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).