Architecture review

Before starting the engine rewrite, we need to rethink the current architecture. I mean reorganizing components and be more modular.
After that rework, it will be way simpler to think about the sync engine rewrite.

Goals are:

• Each component must work on its own and not be dispatched in others components.
• A component can be optional and pluggable.
• Easier Python 3 migration by compatibility checking (verifying a single module is more doable than the whole software). (no more revelant since NXDRIVE-691)

For instance, let's take the Auto-Lock feature:

• Ideally it should be a lone module that we decide to import and activate.
• But currently, the feature is splitted into the Auto-Lock, Drive Edit, Notifications and DAO components. It is hard to understand and modify.

Components list:

• Sync engine (including watchers)
• Direct Edit
• Direct Transfer
• Auto-Lock
• GUI
• CLI
• Options
• Database
• Notifications
• OS integration
• Updater
• Installer