refactor: project structure refactor

[matthew-hagemann]

First pass at refactoring the file structure to match our new project layout without moving any code around.

Some things to note in firmware_updater:

• Allot code is shared across multiple files, making me decided not to create sub directories within pages, services etc. For example:
  • pages/device_store exports DeviceStore used in all pages
  • pages/device_model exports DeviceModel used in all pages and in multiple widgets
• I am not 100% sure where to place lib/app/fwupd_x.dart. It is used in multiple pages, services and widgets, as is lib/app/fwupd_notifier, which made it feel like the belonged in app. Perhaps common?
• Not all names align with our new naming scheme. Should I do some name refactoring aswell?

[sminez]

Reminder for myself and/or @d-loose: we'll need to update the config in weblate when this merges.