Divide the shroud selector into groups.
Add flags and player colours to the observer selector.
Add LogicKeyListenerWidget for binding keys to UI logic.
Move control group logic into its own logic class.
Hook up observer view keybindings. Fixes #4435.

Uses 0-9 to select view (plus shift to cycle backwards), '-' for                 combined shroud and '=' for world view.
Update changelog.
Merge pull request #4942 from pchote/more-replays

Overhaul observer shroud selector.