License clarification

[FedericoCeratto]

Hello and thanks for developing Tooth.
According to license_type in src/Application.vala and the badge in the README.md file Tooth is strictly under GPL v3.
According to data/dev.geopjr.tooth.appdata.xml.in it is under GPL-3.0+ meaning that users can pick any future version of the license at they choice.

I understand that those files might come from @bleakgrey 's repo but this little conflict should be addressed anyways if possible.

(As a side note, it is considered a good practice to add a small comment at the top of each source code file to indicate authorship and license)

Thank you!
Related to #27

[GeopJr]

I probably don't have a say on this but here's the blames:

Initial commit is "or later": bleakgrey/tootle@30f8348#diff-248a3ad2376574c96e7b8fc79764c5a217d4d41c8bf8679bcca1c3b7b87f8ff2R7

Granite drop is "only": bleakgrey/tootle@748808d#diff-94f3624867a327aa824c771318c113fe3a29c5149637a70aac408b786bf2ee35R15

(I'll consider license headers on each file!)

[FedericoCeratto]

Also this is the addition of the "only" in src/Application.vala https://github.com/bleakgrey/tootle/pull/310/files#diff-f4e5d48d751c670a8575715da1cb0a0976532427a8b6143c7fdede415a33ba6cR180

In my understanding regardless if @bleakgrey meant to use GPLv3 or GPLv3+ you are given the right to select GPLv3 [if that's what you want]

[GeopJr]

I went with GPL-3.0!

[Newbytee]

GPL-3.0 and GPL-3.0+ are deprecated identifiers according to SPDX: https://spdx.org/licenses/

GPL-3.0-only and GPL-3.0-or-later are preferred.

[GeopJr]

Done! (Changed to GPL-3.0-only)

[Newbytee]

@FedericoCeratto is there something wrong with my suggestion? Why did you downvote my comment?