Add gain control support to IDS uEye camera driver

- Added 'gain' property to gstidsueyesrc element (0-100, 0=auto)
- Implemented hardware gain control using is_SetHardwareGain() API
- Added --gain/-g command-line argument to launch-ids.py
- Added SET_GAIN and GET_GAIN UDP control commands
- Updated STATUS command to include gain value
- Added get-gain and set-gain commands to camera_control.py test client
- Gain can be set at startup or adjusted dynamically during runtime

sys/idsueye/gstidsueyesrc.h

@@ -57,6 +57,7 @@ struct _GstIdsueyeSrc
   gint timeout;
   gdouble exposure;
   gdouble framerate;
+  gint gain;
 
   GstClockTime acq_start_time;
   guint32 last_frame_count;