Revert part of hrev24647

The baudrate constant for MIDI speed was after all the others in BeOS,
and we have to keep them with the same values for things to work.
Moreover, the constants in SerialPort.h were not changed, so everything
was out of sync and all apps using BSerialPort ended up using the wrong
speed.

Add a comment in termios.h to make sure this doesn't happen again.
SerialConnect: fix baudrate settings save.

There was a mixup of using the baudrate value (eg 115200), constant
(B_115200_BPS = 17) and position in the menu (18), leading to the
baudrate setting not being saved and restored properly.
SerialConnect: fix rdef file

* Declare icon the right way,
* Also add version and app signature.