Long error messages shouldn't resize the window
Stack should always touch the bottom of the window
Stack should be visible on any color bg
an undo command would be nice
command history, too.
string_to_float uses strtod to convert the input ... don't know why strtold
	doesn't work
'OK' button in help should be automatically selected
nicely handle input lines with syntactic errors (probably ought to hold the
	calulator's state until the full line is parsed)
handle unterminated strings correctly
