Improved font handling:

Improved font handling:

 . Added ability to parse a string (e.g. "Courier New bold underline 20")
   into a font.
 . Use this to parse MITSCHEME_FONT.  Removed MITSCHEME_FONT_SIZE env
   variable.
 . Added primitives WIN32-SCREEN-SET-FONT and WIN32-SCREEN-SET-DEFAULT-FONT

Improved window sizing.

  Previously, if the main window was launched minimized, it woudl
  believe the main window should be the size of the icon, about 3x3
  characters: too small to hold an error message.  Now it figures out
  or guesses what the size and position would have been if and
  initializes to that.