feat(gui): Add GUI to interactively use the design tool

BREAKING CHANGE:

Class variables in mainclass.py are refactored to only indicate
accessible ones being public.

A new config module is integrated to save data for the next startup.

A new option "-g" is available to start the GUI from the command-line.

For installation, setup.py configuration is extended to enable py2app to
generate an OSX application.

Included icons with GPL-3.0 license.

The cgi.html consists of JavaScript written code to pass data to the
cgi_script.py and is included to facilitate web-based CGI usage.

The structure of the mainclass.py and mixin classes is slightly changed
for redundant code removal and better readability.

The README file has been updated accordingly.

README.rst

@@ -58,9 +58,9 @@ If you find this work helpful, please cite the following publications:
 
 .. Image substitutions
 
-.. |release| image:: https://img.shields.io/github/release-pre/hahnec/plenoptisign.svg?style=flat-square
-    :target: http://www.github.com/hahnec/plenoptisign/releases/download/v0.4.0-alpha/plenoptisign.zip
-    :alt: Pre-release
+.. |release| image:: https://img.shields.io/github/release/hahnec/plenoptisign.svg?style=flat-square
+    :target: http://www.github.com/hahnec/plenoptisign/releases/download/v1.0.0-beta/plenoptisign.zip
+    :alt: Release
 
 .. |license| image:: https://img.shields.io/github/license/hahnec/plenoptisign.svg?style=flat-square
     :target: https://www.gnu.org/licenses/gpl-3.0.en.html