drawspaces: split app activatable into its own file
drawspaces: rename plugin to window-activatable
drawspaces: move register_types to app activatable
drawspaces: move peas configurable to app activatable

It makes more sense to have it in the app
drawspaces: make the window inherit from GObject
drawspaces: no need for public private in app activatable
drawspaces: use new private accessor way for window activatable
drawspaces: create/unref settings from de/activate
Update POTFILES.in