Changes mainly to support the impoved Edwin Win32 Screen.

Changes mainly to support the impoved Edwin Win32 Screen.

 . Eliminated WM_ERASEBKGND.  Changed WM_PAINT to paint in the pixels
   near the edge that WM_ERASEBKGND.  This change is more efficient
   and avoids flicker on resizing.

 . One icon per screen rather than one for all screens.

 . Key and mouse messages are not converted into input events when
   window is iconic

 . Added a focus-tracking device for use by Edwin.  Re-worked Focus
   and Caret code to get it right.

 . Window text color defaults to the system default rather than black.

 . New mode SCREEN_MODE_VK_KEYS controls whether arrow key events are
   sent as special keys or emacs-ascii (^P for up etc).

 . Minimum screen size is 3 rows of 5 columns, pending rework of edwin
   to advise screen sizes