New: Add GL menus to viewers for platforms not supporting WinForms

[leezer3]

WIP implementation of the GL menu to the viewers, in an attempt to improve general UI here on systems not supporting WinForms.

Only currently in Object Viewer, and it doesn't yet do anything, but the main drawing routines etc. now work.

Probably going to do a little re-design in the menu implementation too.

Primarily relevant for OS-X 64-bit (which is most Apple machines now) https://github.com/leezer3/OpenBVE/issues/1023