Make members in LineNumberMargin protected so derived classes can specialize it and still use the same typeface, emSize & maxLineNumberLength

This helps in customizing the line numbers margin which is what I ran into and had to hack up things. 
This link was where I took suggestions from and ran into problems trying to replicate the same functionality for OnRender where I wanted the logic to be the same as the original class but just change the line number values.
http://community.sharpdevelop.net/forums/t/11706.aspx