Add missing license headers

[danirabbit]

Some of the files with existing headers were licensed as LGPL so that's what I did here. Let me know if that should be GPL instead

[lenemter]

Shouldn't we use SPDX-FileCopyrightText before 'Copyright 2025 elementary, Inc. (https://elementary.io)'?

[danirabbit]

Oops, yes you're right. Bad copy/paste. Should be fixed now

[stsdc]

Some files have LGPL, because they're straight from elementary's projects. I left header files to respect original license.
Is there a need for header files if there is LICENSE file in a project root directory?
Also headers are LGPL and LICENSE file is GPL, isn't that a conflict?

[danirabbit]

@stsdc okay changed those all to GPL instead of LGPL, except for the ones that were previously already licensed as LGPL.

It's not required but it is recommended. Especially when you have multiple licenses in a project, such as we do here were some files are LGPL. It just makes things more clear and easier for someone to respect the license if they re-use this code