libdeng2: Added System class, moved Event to core
Refactor|libdeng2|libshell: Events are passed by reference to widgets
libdeng2|libgui: Application subsystems and GuiApp's refresh

Any number of subsystems can be added to an App.

GuiApp will advance clock time as often as possible (unless a larger
interval requested).
Refactor: Added ScriptSystem, relocated modules there (from App)

It is no longer App's responsibility to deal with anything related to
scripts (apart from owning a ScriptSystem).
Cleanup
Refactor|libdeng2: FS renamed to FileSystem, derived from System

The file system is now a proper subsystem of the application, and
will receive app time updates for time-based processing.
Cleanup
libdeng2|ScriptSystem|Fixed: Adding native modules

Cannot refer to the private instance before it has been assigned to d.
Documentation|libdeng2: Clarification about System
Refactor|libdeng2: DENG2_PRIVATE macro to further aid pimpl idiom use
Refactor|Server|Client: Added ServerSystem, splitting sys_network between apps

The goal is that the client's sys_network contains client-only
functionality, while on the server, ServerSystem handles everything
and sys_network is entirely absent.

Work in progress…

Todo for later: Get rid of LegacyNetwork.
Refactor: Separating low-level networking, removing unnecessary code

The network is always available and initialized automatically.
Fixed|Server: ServerSystem init and shutdown

Presently ServerSystem is dependent on LegacyNetwork, so it must be
deleted before LegacyCore is deleted.
Refactor: Began cleaning up network subsystem headers
Client: Cleanup
Refactor|Server: Cleanup, removed N_TerminateNode()
libdeng2: Imported the Id class from Hawthorn

Unique 32-bit identifiers.
Refactor|Server: Removed use of LegacyNetwork, added RemoteUser

The RemoteUser class handles communication with a connected socket.
Server: Cleanup
Cleanup