- undo
 - shortcuts for keto,spiro etc. ?
 - decide on file suffix, present only these files in file dialogs ?
 - print button (probably will just do postscript export piped to lpr)
 - more demo structures / templates
 - write own output module for postscript ?
 - better documentation
 - use a configuration file for user-specific settings such as window size,
   data directory, print command, fonts, bond length, etc. 
 - use LaTeX-like brace syntax for multi-character sub- and superscripts
 - add keyboard support (especially for the cursor keys)
 - add support for drawing short bonds of fixed length (1/3, 1/2, 2/3) 
 - improve 'white-out' effect around labels - the current round shape looks
   funny on wedge-shaped bonds
 - allow different text sizes in a drawing
 - read pdb, read/write mol format