Use a white version of the Syphon icon in the GUI

The full-colour Syphon logo stood out among the white toolbar glyphs, so use
a white treatment of it (luminance->alpha, brightened) for the "Add Syphon
Source" button and Syphon source items, matching the rest of the toolbar.
The colour version is no longer referenced and is removed.

Author: aalex

resources/images/icons/syphon-white.png

@@ -10,7 +10,7 @@
         <file alias="add-video">images/icons/add_movie_w.png</file>
         <file alias="add-camera">images/icons/add_camera_w.png</file>
         <file alias="add-color">images/icons/add_paint_w.png</file>
-        <file alias="add-syphon">images/icons/syphon.png</file>
+        <file alias="add-syphon">images/icons/syphon-white.png</file>
         <file alias="add-mesh">images/shapes/add_quad.png</file>
         <file alias="add-triangle">images/shapes/add_triangle.png</file>
         <file alias="add-ellipse">images/shapes/add_circle.png</file>