Add load/save config facility

[semuadmin]

PyGPSClient Pull Request Template

Description

Add facility to save and reload GUI configuration. Access via File...Save Configuration and File...Load Configuration. Default config filename is $HOME/pygpsclient.json and any such file will be automatically loaded on start.

Testing

Please test all changes, however trivial, against the supplied pytest suite tests/test_*.py. Please describe any test cases you have amended or added to this suite to maintain >= 99% code coverage.

Checklist:

• [x] My code follows the style guidelines of this project (see CONTRIBUTING.MD).
• [x] I have performed a self-review of my own code.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] (if appropriate) I have added test cases to the tests/test_*.py unittest suite to maintain >= 99% code coverage.
• [x] I have tested my code against the full tests/test_*.py unittest suite.
• [x] My changes generate no new warnings.
• [x] Any dependent changes have been merged and published in downstream modules.
• [x] I have signed my commits.
• [x] I understand and acknowledge that the code will be published under a BSD 3-Clause license.

[semuadmin]

Suggest following enhancements:

1. add command line option to specify config file path e.g. --config

2. allow for mqapikey, mqttclientid and colortags to be specified in config file, and maybe deprecate use of discrete files in future versions?

Done

[semuadmin]

moved to RC 1.3.22