Add check for SPDX license identifier

[mzfr]

Fixes #126
Should this be reported as a WARNING or just INFORMATION?

[razzeee]

Not too excited about this, as it will generate a flood of messages either way.

[mzfr]

Yes this will but another option is just say something like SPDX IDENTIFIER MISSING or one another thing I can think of, combine all the name of the files and print it like

SPDX Identifier missing from addon.py,addon1.py,addon2.py

[mzfr]

@razzeee I'll add a check for license tag in addon.xml file but that would still generate lots of warning.

Should we ignore files if the tag is present in the addon.xml assuming developer knows about the identifier thing?

[MartijnKaijser]

License tag should already be mandatory. What this should do is see if doesn't match the SPDX format and suggest it. Same for the python files.

[mzfr]

@razzeee Is there any other changes required in this PR?

[Rechi]

This forces every add-on to be GPLv2+.
I don't think that was your intention.

[mzfr]

oh Yeah, I think I just did what was given to me as an example. Without keeping in mind that XBMC also have MIT, LPGL, BSD etc.
I'll fix this

[mzfr]

@Rechi There's a problem with the identification of Licenses, lot's of addon have GNU GENERAL PUBLIC LICENSE Version 3, 29 June 2007 this type of text in their license tag. Now should I follow the identifiers in https://spdx.org/licenses/, doing that will generate an error on the type I mentioned above or should I just check that the license tag is non-empty, but doing this will pass all types of Licenses including any random string in the license tag.

One solution could be that we create a dictionary of all the license that we need and then use it.

[Rechi]

As I already mentioned at another PR, first require the licence tag to be listed exactly once and have a non empty string as value.
Also there is no reason to disallow non standard licences.

[razzeee]

Do we really want these two checks?

a) Not looking forward to having to have a license string in every file
b) Shouldn't be forcing a GNU license, like rechi correctly mentioned