Thanks Roger, for the feedback especially the licence part. I have least idea about licences but will take care in future.

One question: Is it right to change IdleConf on Ctrl+ and Ctrl- ? I mean to change font for all open windows?
It happens with most text editors but not with browsers.

To remove those 2 hacks for CodeContext and LineNumbers, what we can do is write a ResetFont() for each extenstion and can call it from the ResetFont() of editor window like this: 
for extension in self.extensions:
    extension.ResetFont()
so that we don't have to write seperate font-size hacks for each extension while resetting the font.