Ability to select camera number in OpenCV plugin

When you select a different camera, the plugin should test if it can grab
a frame from the camera, and if not, revert back to the old camera
number.

This should allow switching between two connected webcams with the OpenCV
plugin, not just using the default webcam. (Often the default webcam on a
laptop is the built-in camera, not useful for beam profiling.)

beams/Webcam.py

@@ -2,6 +2,7 @@
 from cv import CV_CAP_PROP_FRAME_WIDTH as FRAME_WIDTH
 from cv import CV_CAP_PROP_FRAME_HEIGHT as FRAME_HEIGHT
 import numpy as N
+from traitsui.api import Item, Label, RangeEditor, VGroup, View
 
 from Camera import Camera, CameraError
 
@@ -34,6 +35,16 @@ class Webcam(Camera):
         'copyright year': '2011',
     }
 
+    view = View(
+        VGroup(
+            Label('Select a different camera number if you\n'
+                'have more than one webcam attached.\n'
+                '-1 is the default camera.'),
+            Item('camera_number',
+                editor=RangeEditor(mode='spinner', low=-1, high=10)),
+        ),
+    )
+
     def __init__(self, **traits):
         super(Webcam, self).__init__(
             id_string='OpenCV driver, unknown camera',
@@ -60,6 +71,14 @@ def query_frame(self):
             raise CameraError('Could not query image', self.camera_number)
         self.frame = ipl2array(iplimage)
 
+    def _camera_number_changed(self, old, new):
+        try:
+            self.open()
+        except CameraError:
+            print 'Camera', new, 'was not found or could not be initialized.'
+            print 'Changing back to camera', old, 'instead.'
+            self.camera_number = old
+
     def _resolution_default(self):
         '''Resolution of the webcam - a 2-tuple'''
         width = cv.GetCaptureProperty(self._capture, FRAME_WIDTH)