Initial multi-HUD support.

Create a separate HUD for each local human player and set the layout.
Stack split-screen, to match Observer.
Set splitscreen mode for observer.
Promote HudCell from Hud into Display.
Define operator<< for HudCell.
Use HudCell in Observer.
Adjust HUD size based on cell.
Always fill the screen, even with 3 players.
Hook started/finished signals to MetaPlayer.

SessionPeer now hooks up the started and finished signals directly from
MainCharacter to MetaPlayer, instead of GameScene doing a check on each
frame.

This fixes the started/finished events not being fired on other than the
first player.
Fix finished signal possibly firing more than once.
Remove unused var.
Transition to POSTGAME and DONE phases.
Reformat.
Pass the HUD scale to the decors.
Set HUD scale on decors when added.
Set HUD scale.
Remove debug logging.
Set HUD scale.
Document OnHudScaled().
Clean up conversion warning.