clipbrd: minor improvements

The patch does the following:

• Display the application icon in program's about dialog
• Add missing DT_NOPREFIX flag to DrawText() calls
• Let ShowLastWin32Error() show an error message even if the last error code is zero
• remove the calls to OpenClipboard() and CloseClipboard() from the functions in winutils.c and let the caller open and close the clipboard instead