Generify background.js Phases 1,2: Use the manifest for non-tabular data

[eyalroz]

Let's try to make background.js a completely generic file, which relies on declarative specification, by the extension, of its contents, and loads it.

This will hinge on getting data from the package manifest, in manifest.json

This should have three phases:

1. Make the manifest usable in background.js
2. Use the manifest to obtain/generate non-tabular data, like the options dialog URI and the default pref file path
3. Use the manifest to obtain/generate the tabular data passed to WindowsListener: The per-window overlay injectors, and the Chrome URLs for registration. That means the tables will go into the manifest.json, in some form or another - or alternative, will be generated from inspecting package files and directories.

Once this is done, we might take a crack at dropping the overlay injectors altogether and making that part declarative as well.

Anyway, this bug is about phases (1.) and (2.)