bugfix pydev-editor's icon when in dark theme.

The python_file icon seem has wrong transparency when the background is dark; 

current : 
![bug](https://raw.github.com/x2nie/Pydev/iconic/extra/python_file_icon_bug.png)

when patch applied :
![bug-fixed](https://raw.github.com/x2nie/Pydev/iconic/extra/python_file_icon_bugfixed.png)