New feature: speech_name for audible alerts

Added new feature to have a speech name for an aircraft. Currently, with the -speech option on the gcs, it reads out the aircraft name prior to the alert. This takes considerable time when an aircraft name is something like UAV_3_Config_B, and reduces the effectiveness of audible alerts. As such, one can now specify a speech_name in the conf.xml entry for the aircraft, which the speech function will use as an alternative. Only letters, numbers and underscores are valid (not checked in code, currently exits GCS, needs better error handling, maybe checked as ac_name in pc_aircraft.ml). To skip the name altogether, simply use a single underscore. If no speech_name is given, the default is to use the aircraft name, as before. The speech name is what shows up in the alerts tab in the GCS. Note that this was tested on OS X Lion in simulation only, no real tests or replay tests have yet been completed.

It is not yet completed, because paparazzi center does not handle the new field in the xml yet, and it gets deleted as soon as you make a change (like switching a/c in the dropdown). Also need add an example.

Add to the conf.xml like so:
<aircraft
   name="Microjet"
   ac_id="5"
   ...
   gui_color="#6293ba"
   speech_name="My_Plane"
  />

@gautierhattenberger any thoughts on how to fix the paparazzi center side? I want it to be able to handle either having the xml field, or not, and not having to have the field there.

Any thoughts on this and feedback/criticism would be appreciated.