Consider supporting ~/.local/share/flatpak/extension dir

[alexlarsson]

The idea here is that a directory like ~/.local/share/flatpak/extension/org.some.Extension/x86_64/master would be completely unmaintained by flatpak itself (i.e. owned by the user) but it would be used as if a runtime with that ref was installed when resolving extensions for an app/runtime.

This has several interesting uses. First of all, one could use this to make a simple org.freedesktop.Platform.GL implementation with a copy of some host openGL libraries. This could even be scripted (although it has to be done carefully to avoid conflict between host and runtime libs).

Secondly it can be used for apps to support plugins. For instance, gedit could have a plugin extension point, and then the user could just copy plugin files to the right extension dir.

We can even use this for things like versioned themes. If you have gtk+ in the runtime for instance, you can expose an extension point like "runtime/org.gtk.GtkThemes/x86_64/3.20" which points to somewhere in the runtime where gtk looks for themes. It could even be a subdirectory extension point (if we make gtk look in subdirs), which means you can package themes as flatpak extensions as well as supporting the exension dir for users to just copy stuff.

[alexlarsson]

In master now.