Just more example to experiment with various things

When an entity dies, it is immediately removed from the current selection
Only select entities that can be selected

It is used to attach certain response (for example graphical or sound
effects) with various events of the World

With an example of Blink

And use more precise duration types

And clean some time-related stuf

fix #2268

And fix the fog for that, as well

Use the Entity class everywhere, instead
fix #2803

mod/client_mode, and the action panels, mostly

The world is now isometric. fix #2277
Simplify TurnHandler. fix #2724 #2704

The world and game modules are now more decoupled. fix #2587

Implement a debughud. fix #2417

Better separation of modules (network does not know about world anymore, for
example).
Ref #2587