Add LOGFONT struct

[damiansalvia]

I added the LOGFONT struct used by ToLogFont and FromLogFont methods.

I also added a set of related constants defined by GDI (see LOGFONTA structure). They are not used, that's why are defined as internal, except in those Font constructors that receive gdiCharSet parameter (so I removed the FONT_CHARSET private enum that were defined in the Font class and now it uses the GDI_CHARSET static class values).

[cinthamo]

LGTM
I wonder, why are you implementing these functions?
It is not necessary to complete all System.Drawing interface, just respect the part we need. Is this needed for something?

[damiansalvia]

I don't think anyone is using these methods, but just in case, I noticed it doesn't make sense to call ToLogFont or FromLogFont without having a reference to the LOGFONT struct from the System.Drawing library. Additionally, the new GDIDEFS source file, which stores GDI_CHARSET (among others), provides a clearer and more organized location for the possible gdiCharset values (in the Font constructor) instead of defining them in the Font class.