Minor punctuation edits in README.rst.
Edits to AboutDialog and related code.
AboutDialog no longer has buttons, but rather:
static text providing developer and artist credits; and
a brief reference to Apache License, Version 2.0, with a hyperlink to
the text of the license on the Apache website. (This seems more
maintainable than installing the license on the user's machine.)
License no longer installed on user's machine.
Added a TODO re. text positioning in AboutDialog.