Add Uninstall Entrypoint

[XedinUnknown]

The Problem

Many plugins have an e.g. uninstall.php entrypoint, which allows the plugin to be uninstalled (e.g. have its data removed, cache cleared, rules flushed, etc) when a plugin is uninstalled. It is therefore a good idea to have one bundled with the bootstrap, but none are currently available.

Suggested Solution

1. Allow easier bootstrapping of the same application with different sets of modules.
2. Add an uninstall module. Loaded last, it would be able to override any other configuration, replacing any extra services with perhaps no-op ones (e.g. to eliminate dependency on other plugins during uninstall), and to trigger an uninstall event that other modules could react to by performing their uninstall procedures.
3. Load the uninstall module last, and only in the uninstall entrypoint.