Connect/disconnect/reconnect from console; major Client refactor

[cormac-obrien]

• Refactored ClientState into its own module and delegated most game-state-only functions to it
• Added Connection object which now owns ClientState, as the game world only exists when there is an active connection
• Collected most rendering functionality into new ClientRenderer object
• Folded most code from quake_client::game into Client and Connection
• Fixed a bug which prevented bf command from properly flashing the screen

And, most significantly,

• Enabled disconnected state, which is now the default when the game is started (pending automatic demo playback with playdemo in quake.rc)
• Enabled console control over connection with connect, disconnect and reconnect commands

[cormac-obrien]

bors r+

[bors[bot]]

Build succeeded:

• continuous-integration/travis-ci/push