Changing the MIT license to GNU GPL-3.0

[Taitava]

Discussed in #192

A TODO list for me

A quote from the GNU GPL-3.0 license:

An interactive user interface displays "Appropriate Legal Notices"
to the extent that it includes a convenient and prominently visible
feature that (1) displays an appropriate copyright notice, and (2)
tells the user that there is no warranty for the work (except to the
extent that warranties are provided), that licensees may convey the
work under this License, and how to view a copy of this License. If
the interface presents a list of user commands or options, such as a
menu, a prominent item in the list meets this criterion.

Add a license link to:

• The plugin's settings panel.
• README.md
• Each source code file.
• The generated main.js file. (Find out how).

Add a notice about no warranty to:

• The plugin's settings panel.
• It's already mentioned in README.md.
• Each source code file.
• The generated main.js file. (Find out how).

Other stuff:

• Change the LICENSE file.
• Change the license in package.json.

[Taitava]

Done.

[Taitava]

Released.