Draft: Feature/libportal

What does this MR do and why?

This MR adds the libportal library as bindings:extra:libportal

Still TODO:

• Fix the libportal gradle version config
• Move the portal demo code into a dedicated sample module
• Add XdpGtk4-1.0.gir as a separate gir/module to have Gtk4 Parent window support.

MR acceptance checklist

• I have rebased this branch on top of the destination branch (usually master)
• I have executed locally ./gradlew spotlessApply check before creating the MR and it has run successfully
• My contribution is fully baked and ready to be merged as is
• I have performed a self-review of my own code
• There are no WIP commits in this PR
• I ensure that all the open issues my contribution closes are mentioned in the commit message using the Closes #1234 syntax