Provide a more packager-friendly way of installing appdata files

[jurf]

The thing is if an appdata file is missing, one has to add it as a source and then either modify build-commands or post-install and add something like "mkdir -p /app/share/appdata", "install -Dm644 appdata.xml /app/share/appdata/appname.appdata.xml" and add a "rename-appdata-file": "appname.appdata.xml", but with this approach a simple appdata change requires a rebuild, and, frankly, the approach is a bit tedious.

It would be cool if there was something like:

      "sources": [
        {
          "type": "appdata",
          "path": "appdata.xml"
        }
      ]

which would perform all the above, automatically, without needing a rebuild.

[TingPing]

This could be even more generic like:

"sources": [
  {
    "type": "file",
    "path": "foo.Bar.appdata.xml",
    "install-directory": "/app/share/appdata"
  }
]

[alexlarsson]

Could have optional permissions too, as well as support for source filename and destination filename being different.