Merge plugin-toggles feature branch
This branch includes the following changes (previously reviewed when they were merged to the feature branch):
- Add a switch to the settings screen for enabling/disabling each plugin (for Tor and Bluetooth this involves refactoring existing settings)
- Add IPv6 support to the LAN plugin
- Improve the plugin API
- Show an amber icon when the Tor plugin is starting
- Fix various small bugs we found while working on this feature
I'd like to get these changes merged to master, as it's tricky to write plugin-related code based on master without creating merge conflicts with this branch. I think the UI changes in this branch are an improvement on the current UI and are worth merging even though the final UI for controlling plugins isn't settled.