Relicense some code files from GPL to 3-clause BSD

This was requested by the Elektra project (https://github.com/ElektraInitiative/libelektra)
in order to be able to use this code.
Merge branch 'feature/relicense-code' into develop
Refactor JSON configuration file module

Generalize the JsonSnippetFile into a JsonFile class that reads or saves
a JSON file and uses JsonTranslatorFactory to retrieve a JsonTranslator
class that can handle the specified item type stored in the file.
Merge branch 'develop' into wip/custom-styles

Conflicts:
	app-static/json/jsontranslatorfactory.h
	app/mainwindow.cpp
	test/unit/jsontranslatorfactorytest.cpp
	test/unit/jsontranslatorfactorytest.h
	test/unit/main.cpp